The following essay is adapted from The MVP Machine: How Baseball’s New Nonconformists Are Using Data to Build Better Players, out now from Basic Books.

April 16, 2018, was an off day in Los Angeles for the Boston Red Sox. But for outfielder Mookie Betts, there would be no rest.

Boston hitting coach Tim Hyers suggested to Betts that he spend the day working with the silver-haired, blue-eyed Doug Latta, a self-described swing whisperer. On that Monday morning, Latta drove to the team hotel and picked up Betts, who brought with him a couple of his personalized Axe Bats, which feature handles shaped like that of a hatchet. They drove to a facility Latta occasionally used that was closer to Anaheim than his own Los Angeles headquarters: the covered batting cages at Anteater Ballpark, home of the UC–Irvine college baseball team, whose coach was a friend.

The 25-year-old Betts was coming off an excellent 2017 season, albeit a disappointing one for him. His .803 OPS (on-base plus slugging) was slightly above average but down significantly from his .897 mark the previous season. Many athletes are afraid to change. Betts wasn’t, even if that meant listening to the advice of an outside instructor like Latta, whose own playing career never advanced past junior college. Already a two-time All-Star, Betts wanted to be better.

Many of the hitters who had previously sought change were desperate, seeking to extend careers. But what if a top-shelf talent like Betts—renowned for his elite hand-eye coordination and athleticism and already experimenting with a nontraditional bat handle designed to enhance bat speed and control—adopted a better swing? What if stars started rethinking their potential and making large leaps?

Hyers joined the Red Sox after the 2017 season, following a two-year stint as the assistant hitting coach for the Dodgers. A Georgia native with a trace of a Southern accent, Hyers had a brief big-league career, batting .217 with two homers in 133 career games for the Padres, Marlins, and Tigers. He had been taught to swing down on the ball. He didn’t think he could be a power hitter. He thought his skill level was fixed.

Hyers sought out Latta late in the spring of 2016 after speaking with Dodgers infielder Justin Turner about the swing he learned from Latta. They met for breakfast one morning in the suburb of Silver Lake. The meeting lasted four hours.

Up to that point, few professional coaches had accepted Latta into their circles of trust. He was perceived as an unwelcome outsider who meddled with other coaches’ players. But as Boston’s charter arrived in Los Angeles on that early season road trip, Hyers wanted “another set of eyes”—Latta’s eyes—on a swing Betts had begun to forge that spring. Latta was a master at understanding how every part of the body worked in concert.

As Latta drove Betts to the UC–Irvine baseball facility, he asked him if he was sure he wanted to exert himself. Betts had been out of the lineup Sunday after injuring his left foot in a collision at home plate the day before. He said he was fine, and X-rays were negative. So they went to work as Latta does with almost every hitter. “We want the hand path to work and get length,” said Latta of creating an upswing with extension, “to get the body balanced and move forward.”

They worked together in private for several hours in open-air cages covered by hunter-green corrugated metal. The next day, Betts returned to the lineup against the Angels’ rookie sensation Shohei Ohtani. Betts batted first, where he had taken the vast majority of his at-bats as a big leaguer. At 5-foot-9, 180 pounds, he had the body of a leadoff hitter in addition to elite hand-eye ability. He had shown surprising power. That threat was about to become even more significant.

In the Anaheim twilight, Ohtani threw a full-count fastball that came close to the plate, knee-high. A keen observer could have noticed that Betts’s swing—already much changed that spring under Hyers’ instruction—looked different. As Ohtani delivered, Betts lifted his left leg and began his stride toward the pitcher. His hands dropped lower, toward his belt. As he moved forward, his hands stayed back and “under” the ball, as Latta teaches, which would enable his bat to travel on an uppercut trajectory. Pitches travel at downward angles, both because they’re released from an elevated mound and because gravity acts upon them. A flat bat path is on plane with a pitch for a very short period of time. An upward path increases the odds of optimum contact.

In a blur, the barrel connected with the Ohtani pitch and drove it high into the violet sky, the silhouette of the San Gabriel Mountains still visible in the distance. Rather than wrapping around his back as it often did, Betts’s follow-through traveled on a path that finished high and slightly above his shoulders. It was almost like a golfer’s swing—or like Turner’s.

The ball traveled 411 feet, landing beyond the left-center-field fence and ricocheting off the faux boulders and back onto the field. Betts ran around the bases, businesslike. No smile cracked his countenance.

When Betts came up in the third inning, Luke Bard had replaced Ohtani. Bard’s second pitch to Betts was a hanging slider. Betts again dropped his hands lower. His bat got on plane with the pitch earlier, and he drove slightly under and through the ball with lightning-quick hands. He again finished higher in his follow-through. Betts launched the ball beyond the bullpens in left-center field, 417 feet away from home plate. He lowered his head and subtly flipped his bat.

In the eighth inning, Betts faced Cam Bedrosian. As the temperature dropped throughout the evening, Betts blew on his hands prior to grasping his Axe Bat and stepping back into the right-handed batter’s box. Bedrosian’s first pitch was an inside fastball. Betts swung, making contact well out in front of the plate, where most home-run contact happens. The ball soared toward center field. This time Betts watched it briefly, and a small grin appeared. The ball landed on an AstroTurf knoll beyond the playing surface, 427 feet away: his third home run of the game. And on May 2, Betts would author another three-homer game.

Betts was following the trail blazed before him by a select group of hitters, including one Latta had helped transform from a castoff into a star.

On Sept. 6, 2013, Turner, then a New York Mets utility infielder, borrowed the bat of teammate Lucas Duda and walked into the on-field batting cage at Progressive Field in downtown Cleveland. It was the beginning of one of the most unthinkable transformations in modern baseball history.

Turner’s former Mets teammate, Marlon Byrd, had been traded a week earlier to the Pirates. Turner and Byrd had spoken often about hitting. The public was suspicious of Byrd’s 2013 breakout because Byrd had been suspended for 50 games in 2012 after testing positive for a banned substance that masks steroid use. (Byrd would retire in 2016 after testing positive for a banned substance again.) But Turner—and another light-hitting Mets infielder named Daniel Murphy—listened to Byrd in 2013 because Byrd had changed his approach to hitting in a way that had nothing to do with drugs.

Byrd was 6 feet tall with a 245-pound frame, and yet prior to 2013, he had been a ground-ball hitter. “I came from more of an old-school style of baseball,” Byrd told Travis in 2013. “Coming up, the coaches I had played for in the 1970s and 1980s, they were taught to swing down.”

Byrd’s breakout 2013 season, which featured career-bests in home runs (22) and slugging percentage (.526), came at an age, 36, when players are typically in decline. His preceding suspension made the stats look like a chemical mirage, but in a single offseason, Byrd had changed the nature of how he swung and the angle at which his bat struck the ball.

Byrd and Turner conversed during batting practice, during games, and on late-night charter flights as they crisscrossed the country to fulfill the demands of the major-league schedule. Turner was intrigued by Byrd’s radical new theories of hitting. Byrd spoke about how Latta had helped him adopt a leg kick and more loft in his swing the previous winter. As a result, he was more direct in his movement to the pitch. Turner knew Byrd had decreased his ground-ball rate from 49.6 percent in 2012 (which was near his career rate) to 39.2 percent in 2013, while raising his fly-ball rate by 12.2 percentage points, the third-greatest increase in baseball. Professional batters take tens of thousands of swings between when they first pick up a bat and when they make it to pro ball, hardwiring a swing path into their muscle memory. Most coaches thought that trait couldn’t be changed. Byrd challenged that notion, suggesting that hitters could evolve, and relatively quickly, even at advanced ages.

Byrd embraced a style completely different from what Mets hitting instructor Dave Hudgens advocated. (Hudgens would later adjust his beliefs when he became the Astros’ hitting coach.) Early that spring, Byrd launched ball after ball out of a spring training backfield at the club’s Port St. Lucie complex. The Mets’ staff told him his new swing wouldn’t work in games. He was sure they were wrong. Hitters were taught that home runs were blissful accidents, and that they ought to try for low line drives. Byrd believed in trying to hit home runs in games and in practice. He was done with conventional wisdom.

Turner had not homered in his 183 plate appearances in 2013, and he started Sept. 6 with a .639 OPS, well below the league average. He was a bench player known for his glove, shock of red hair, and amber beard. But on this midafternoon in Cleveland prior to a 7:05 p.m. game, Turner tried on a new identity.

Duda’s bat was an inch longer and an ounce heavier than his own model—34 inches, 33 ounces—which is why Turner chose it. Maybe it would help him generate more power. As he entered the on-field batting cage to begin his experiment, he thought about what Byrd had preached: “Take a large stride. Gain ground.” In other words, move toward the pitcher, creating a more linear path that would transfer energy more efficiently. Byrd also harped on contact point, explaining that batters should think about catching the pitch out in front of the plate, whereas most traditional teaching held that batters should let the ball travel longer and deeper, closer to the plate. Catching the ball out front would better organize the body and allow for the ball to be pulled into the air: the most valuable batted-ball type in baseball. In 2018, MLB batters hit .565 with a 1.267 slugging mark on pull-side air balls, and 32.7 percent of such batted balls went for home runs.

All his life, Turner had been told to let the ball travel as deep as possible and to try to hit low line drives. Now, Turner, who would turn 29 in November, was ready to experiment. He knew the Mets might not tender him a contract after the season. If he wanted to play more, or at all, he had to change. He had to hit.

All his life, Turner had been told to hit low line drives. Now, he was ready to experiment.

Turner had studied kinesiology at Cal State–Fullerton, where he hit seven home runs in 1,008 at-bats. While Byrd’s theories made sense, Turner didn’t think he could become a home-run hitter. He had hit just six home runs over parts of five major-league seasons. But unlike Betts, he had little to lose. He traded in a fixed mindset for a growth one.

“I was thinking, I’m just going to try and catch the ball as far out as I can in batting practice,” Turner says.

Only a few saw the beginning. They saw Turner connect with pitches that began to fly over the 19-foot, left field wall. The clank of balls against empty bleachers resonated throughout the mostly empty ballpark. It was a sound Turner was not accustomed to creating. Teammates raised eyebrows.

“I didn’t even feel like I was swinging hard,” Turner says. “I was like, ‘this is amazing.’ ”

In his first two at-bats on Sept. 6, Turner grounded out versus Scott Kazmir. But in the top of the seventh, he faced right-handed reliever Cody Allen. A paltry crowd of 15,962 watched as Turner caught an elevated fastball well out in front of the plate.

The feeling of hitting a home run was alien to Turner. He ran out of the box. His attention was fixed on Cleveland center fielder Michael Bourn, who raced backward. “Get over his head! Get over his head!” Turner exclaimed in his head, his heart hammering as he approached first base, the ball he had struck still hanging in the air. Turner was as astonished as anyone when the ball vanished into a row of trees beyond the center-field fence.

“I was like, ‘oh my God, what just happened?’ ” Turner says. “I didn’t hit enough home runs to know what it was like when I got it.”

Turner floated around the bases, trying to keep his composure, trying to be emotionless, acting as if he had been there before. Except that he hadn’t been there before. His teammates were slack-jawed. “Swaggy,” as Turner was nicknamed, did that? Following the game, Turner met his then–girlfriend, now wife, at the Jack Casino in downtown Cleveland. Under the lights, above the din of the casino, Turner couldn’t stop talking about what he felt and what he might have found.

Two days later, the Mets and Turner faced the Indians and hard-throwing rookie Danny Salazar on a sunny Sunday afternoon. During his second at-bat, with the count three balls and one strike, Salazar challenged Turner with an elevated 96mph fastball. Turner caught the pitch out in front of the plate. The ball soared on a majestic trajectory, landing halfway up the sun-soaked, left-center-field bleachers. It was as if Roy Hobbs had stolen Turner’s jersey and hit right-handed.

For the eighth time in his career—and the second time in three days— Turner circled the bases.

Turner played in 10 games with the new swing that September. In that limited sample, he batted .387 and slugged .677. He knew he needed to visit Doug Latta.

Amid the urban sprawl of Northridge, California, close to a Costco, a mobile home park, and an In-N-Out Burger, is a long, narrow business park that terminates near a set of railroad tracks. In one of the business park’s long, narrow units, Latta transforms hitters.

Inside Latta’s facility, the Ball Yard, are two batting cages, a couple of couches, a half bathroom, and a kitchenette. Everything one needs to hit for hours. Outside the main batting cage is a large television whose screen Latta has covered with a clear sheet of plastic so he can pause video and use marker to scribble over the image, identifying flaws—his homemade Telestrator. It is a modest, white-drywalled space with a green AstroTurf floor, but as Latta tells hitters who might be expecting a more lavish facility when they first walk in, “We don’t need much space to get better.” The same goes for any hotbed of talent development: All that’s required are ideas, information, passion, and reps. Lots and lots of reps. The Ball Yard was where, in the winter of 2013–2014, Latta helped Turner morph from a journeyman into something more.

At the Ball Yard in 2018, Latta scrolls through his video library, finds some early footage of Turner, and plays it on the flat-screen television outside the batting cage. It’s from Jan. 27, 2014. Turner was unemployed then. He’d been nontendered by the Mets. On the video, Turner has a shorter beard and wears a T-shirt, shorts, and a hat turned backward. He takes a swing. He goes into his leg kick and takes an exaggerated stride.

“This is before he narrowed up,” Latta says of Turner’s stance. “Those days we were just pumping.”

By “pumping,” Latta means taking a lot of swings. Latta pulls up another video of Turner from April 24, 2014, after Turner had signed a minor-league deal with the Dodgers with an invite to spring training—and ended up making the team as a replacement for the recently retired Michael Young. Turner was still a part-time player. In the clip, his hands are higher than they were in January. So is his leg kick. He looks off-balance several times, finishing with his feet out of the would-be batter’s box as he gathers himself. As he swings, he rocks his weight on his back leg, which is a no-no for Latta.

“People forget that’s how J.T. looked then,” Latta says. “He’s gotten narrower, the leg kicks up more. It got easier and easier and easier.”

Latta and Turner weren’t alone. Byrd was often there also, hitting and serving as something of a quasi–assistant coach.

“It was hard. It was tough. I had 25 years of habits to break,” Turner says. “Marlon had talked about it all year long, so I kind of had a head start going in there, knowing what [Latta] was talking about.”

Over that winter, Turner trained at the Ball Yard for three months, four days a week. He and Latta worked for three hours per session, taking seven or eight rounds of batting practice per hour, 15 to 20 pitches per round. He estimates that he took 20,000 swings.

Latta has documented Turner’s entire evolution. Since that winter, most of their interaction has come through texting, though Turner sometimes stops by the Ball Yard for a tune-up. Latta interacts with clients he has less familiarity with through FaceTime. He watches most of Turner’s at-bats live. At first, Latta sent constant reinforcements. If he saw footage of Babe Ruth’s swing on MLB Network or elsewhere, he’d capture it and send it to Turner. Ruth had a narrow stance with an exaggerated stride. He contacted the ball out front. His back foot left contact with the ground. Turner laughed about the Ruth video during spring training 2018.

“ ‘Look at Babe, he’s really staying back and catching the ball deep,’ ” Turner said, recounting the text from Latta that accompanied the video. Ruth did the exact opposite of the conventional teaching Latta and Turner were mocking, complete with an uppercut swing.

“You look at Ruth, Mantle, all these hitters back in the day, they all leg kick or toe tap. They all gain a ton of ground. Their strides are huge. … It’s the theory that whatever starts in motion, stays in motion. … Isaac Newton is my favorite hitting coach. The whole old-school mentality of swinging down, chopping down on the ball? It’s bullshit.”

Turner slammed the table with a fist in frustration. How many coaches had let down how many players?

In 2014, Turner hit .340/.404/.493. He made only 322 plate appearances, but among all hitters with at least 300 trips to the plate, his 158 weighted runs created plus (wRC+), a comprehensive rate stat, ranked ninth (100 is league average). The Dodgers were curious enough to retain the arbitration-eligible Turner on a one-year, $2.5 million deal. In 2015, he made 439 plate appearances and hit nearly as well as he had the year before, slashing .294/.370/.491 with a 141 wRC+. Between 2013 and 2015, he cut his ground-ball rate by 12.5 percentage points, the fourth-greatest decline in the game. In 2016, Turner earned a full-time role. He hit 27 homers. After that season, he signed a four-year, $64 million contract with the Dodgers.

In the summer of 2018 at the Ball Yard, Latta pulls up another evolution-of-Turner video from January 2017. By now, Turner is much narrower in his stance. There is no rocking back on his back leg to move forward. As his left leg lifts to initiate his swing, his hands fall to create the proper slot for an uppercut swing.

“What’s J.T. trying to create? Balance and drive,” Latta says.

Latta plays it again in slow motion. Turner’s back leg and shoulder remain perfectly in line. There is no backward weight transfer or movement. He’s balanced. He lands with his front (left) leg well in front of his body. His hands are lowered and ready to fire. “Body is moving in concert,” Latta says. “From this position he fires right underneath. He picks up his backside, and all that energy drives through the ball. He gets what we call a lot of extension.” In slow motion, the swing finishes. Latta outlines the path of the upswing with marker on the plastic sheet covering the television screen. He notes that all of Turner’s energy is transferred to a contact area in front of the plate.

Latta has a HitTrax radar unit to measure launch angle and exit velocity, but he rarely employs the latest technological advances. If a hitter doesn’t have timing and balance and movement, he has no shot, Latta believes. He’s troubled by the teaching he sees on social media, where video clips focus on creating exit velocity at the expense of balance and adaptable swings. Latta rarely uses the words swing or swing plane when talking about hitters. He tries to organize the body in such a way that the swing takes care of itself. He believes that some in the independent-player-development business are more worried about selling products than helping athletes.

Ground zero of the fly-ball revolution was arguably the Ball Yard, where other teachers and advocates of the movement met and discussed ideas. Byrd spread the knowledge to Turner and Murphy in New York. (Murphy hit 62 homers in more than 3,300 at-bats through age 30, and then hit 60 more in fewer than 1,400 at-bats from ages 31 to 33.) Turner now proselytizes in the Dodgers clubhouse, teaching or reinforcing the beliefs of talented young hitters like Cody Bellinger and Corey Seager, and even influencing some hitting instructors like Hyers. When Bellinger was having trouble with sliders as a rookie in 2017, he wondered if his swing was too uppercut-oriented. Turner had him play a game late that summer. They went into the team’s indoor batting cage at Dodger Stadium. Turner turned on the slider machine and told Bellinger to try to swing and miss below the sliders. Bellinger couldn’t do it; he couldn’t miss. Bellinger produced a .418 weighted on-base average (wOBA) versus sliders in 2017 and a .341 mark in 2018, easily eclipsing the league averages of .271 and .263, respectively. His upswing worked.

The fly-ball revolution also spread thanks to Statcast, which in 2015 began to measure the launch angle and exit velocity of almost every ball hit in every MLB game. Teams started to outfit their batting cages at all levels with swing- and ball-tracking technology. The data confirmed that air balls were better than grounders: With every 10-degree increment from minus 30 degrees to 30 degrees (where zero is a level line drive), the leaguewide wOBA on contact in 2018 increased, easily surpassing the all-angles average of .315. (The average home run left the bat at a vertical angle of 28.2 degrees.)

-30 to -20: .050 -20 to -10: .188 -10 to 0: .245 0 to 10: .462 10 to 20: .712 20 to 30: .731

Between growing awareness of that relationship and a still-unexplained change in the composition of the official MLB ball in 2015, which caused balls in the air to carry farther, hitters had more incentive to swing up. The average launch angle of a batted ball has increased in every year of Statcast: from 10.5 degrees in 2015, to 10.8 in 2016, 11.1 in 2017, and 11.7 in 2018. Over the same span, the rate of balls hit at a 10-degree angle or higher has increased by 3.3 percentage points. According to data from Baseball Prospectus, the leaguewide ground-ball rate in 2018 was the lowest on record, going back to 1950.

In 2017, MLB hitters launched 6,105 home runs, breaking the previous record (set in 2000) by 412. That year, 2018, and 2016, respectively, featured the three highest rates of home runs per fair batted ball in history, surpassing the so-called steroid era.

“Elevate and celebrate” and “Ground balls suck” became batting-cage cries. In 2018, Red Sox skipper Alex Cora said, “We don’t like hitting ground balls. We like hitting the ball in the air.” His statement mirrored a Cubs catchphrase—“There’s no slug on the ground”—as well as Pirates manager Clint Hurdle’s advice to his team: “Your OPS is in the air.” An outsider philosophy had spawned insider slogans.

Latta didn’t set out to change baseball. He simply loved the game. He grew up in what he describes as “tough” financial circumstances and didn’t play organized baseball until he reached high school. Instead he played in a sandlot down the street in his childhood Koreatown neighborhood, where he and his friends used the back of an accommodating neighbor’s garage as a backstop. He went on to play at Los Angeles City College, a public community college, and later Cal Lutheran. After college he joined the Pasadena Redbirds, an adult hardball league.

“The challenge was always there to be able to build myself, to get better, and to realize that you could beat the odds,” Latta says. “We were playing major-league-caliber pitchers—like Ed Farmer, Jerry Reuss, Craig Chamberlain—so we were getting challenged.”

Latta turned down a chance to play in the Mexican League. He already had a better-paying job: At 19, Latta had begun his own business cleaning private swimming pools. He continued to play baseball for the Redbirds until 1995. During his time with the team, he met a professional scout named Craig Wallenbrock. They developed a friendship. Latta describes Wallenbrock as “a thinking man.” Wallenbrock was a devoted reader, and he applied his studies to baseball, giving private hitting lessons beginning in the late 1980s.

“Elevate and celebrate” and “Ground balls suck” became batting-cage cries.

While his playing days were over by the late ’90s, Latta thought he had information to share. He also had space to share. He owned part of a large industrial building in Los Angeles, where he stored pool chemicals for what was then a state-of-the-art chlorine-injection system. When he had an opportunity to buy the other half of the industrial structure, Latta had an idea: He would build an indoor batting cage in the other half of the building. He could give lessons and rent out the cages. He named the facility the Ball Yard.

“We didn’t even have any indoor batting cages in the area,” Latta says. “They just did not exist.”

Latta spent four months personally doing the demolition of what had been office space. He installed AstroTurf and batting cages. Wallenbrock asked if he could hold some of his hitting sessions there. Another instructor was interested in using the space, but that instructor wanted to use “flat-bat stuff,” Latta recalls. “I told myself, hell no.” Wallenbrock was more aligned with Latta’s philosophy, so they reached an agreement for Wallenbrock to train hitters in his space. No hitter was ever taught to chop wood, swing down on the ball, or hit grounders at the Ball Yard, Latta says. Wallenbrock brought in a great number of hitters, including some major leaguers.

The Ball Yard began to host regular meetings with other hitting coaches inside and outside professional baseball, which Latta labels a think tank. Latta kept about 30 folding chairs on hand for the sessions, which grew in size and frequency. The attendees talked about hitting philosophies, some of them out of favor or forgotten like those of the great Ted Williams, who was hitting fly balls before they were cool. “If you get the ball into the air with power, you have the gift to produce the most important hit in baseball: the home run. … For those purposes, I advocate a slight upswing,” wrote Williams in his 1970 book The Science of Hitting. Former major leaguer turned instructor Don Slaught was a frequent visitor, as was Greg Walker, who had served as a hitting coach with the White Sox and Braves. The group met for years.

“I like to say that was my graduate work,” Latta says, “working side by side with Craig for 14 years seeing what [pro hitters] were going through. How do you buy that experience?”

Eventually, Wallenbrock and Latta had a falling-out, and Wallenbrock took his higher-profile clients elsewhere. Latta moved into a new facility, the present-day Ball Yard. Years went by. Latta continued to teach his unusual, uppercut-swing philosophy to players, mostly local college and high-school kids. And one day in the fall of 2012, Marlon Byrd walked through Latta’s door looking for a place to hit in the offseason. Byrd became his first major-league client.

On Feb. 6, 2017, Travis published an article at FanGraphs entitled, “Can More Hitters Get Off the Ground?” Fly-ball and line-drive rates had remained stable from 2012 to 2016, through the depths of baseball’s offensive depression, when a pair of low-power teams in the Kansas City Royals and San Francisco Giants accounted for three World Series titles in five years. In 2014, before the altered ball took flight, teams averaged only 4.07 runs scored per game, the lowest total in a nonstrike season since 1976, and both MLB insiders and fans fretted about how offense could be boosted in a high-velocity era. A few minutes after the story, which quoted Latta, was published online, Latta received a call from Hyers.

“At 7:32 a.m. Tim called me and said, ‘Andrew Friedman just called me about that article,’ ” Latta says. “The president of the Dodgers. Wow. Cool. I had told [Hyers] it was coming out. ‘Friedman got in the office and read it,’ [Hyers said]. I was like, wow, Friedman reads FanGraphs. He had no heads up. For him to get in and read it in two minutes, you never actually know who is paying attention.”

The article created debate within the Dodgers’ hitting circles, which included both more traditionally minded voices like the club’s head hitting coach, Turner Ward, and others like Wallenbrock, who had worked with the club as a consultant, and his understudy Robert Van Scoyoc, who was named as the Dodgers’ hitting coach after the 2018 season. The fly-ball revolution, and the rethinking of development as a whole, had first spread from player to player via word of mouth. Now teams wanted to disseminate it throughout their entire organizations.

For decades, coaches—always ex-professional players—had enjoyed a privileged place in the vertical hierarchy of teaching and decision-making. But more and more coaches were being questioned and challenged. Many were soon to be replaced. Ideas that had begun outside the game were trickling up into the professional ranks. Latta was just one garage startup disrupting the industry.

At the end of 2017, Hyers joined the Red Sox as their head hitting coach. He and the Sox sought to revamp their hitting philosophy. Another new face, J.D. Martinez, was told by the Red Sox coaching staff to make Mookie his “project.” As Martinez watched Betts hit for the first time in the spring with his 2017 stroke, he said, “I don’t know that that’s going to work.” Betts was not offended. He wanted information.

Martinez signed a five-year $110 million deal with the Red Sox over the winter of 2017–2018, several years after reinventing his own swing and saving his career. Through 2013, Martinez had been a replacement-level big leaguer. In his first three years in the majors, he posted an 87 wRC+. He was a below-average hitter and a poor defender. He career was in jeopardy.

“You still talk to coaches, ‘Oh, you want a line drive right up the middle. Right off the back of the L-screen,’ ” Martinez told Travis in 2017. “OK, well that’s a fucking single.”

Martinez began to question why his best swing would result in a “fucking single.” The same winter as Turner, he sought a new swing. He found it in a facility in suburban Los Angeles with assistance from Wallenbrock and his protégé Van Scoyoc.

“I have this little theory,” Martinez says. “When I think about the best players, I think what makes people so good is when they have that insecurity about themselves … because they don’t want to fall off. It keeps them working. I feel like [Betts] had that little doubt. … Other guys have good years and they won’t make the change. He was like, ‘Dude, I have to figure this out. I don’t have it figured out.’ ”

Betts’ overhaul worked from the bottom up, too. Hyers and Martinez first stressed how his feet interacted with the ground. That was where the kinetic chain of the swing began. “I had to fix the ground before I fixed my hands,” Martinez says. “The engine is down there.” The Red Sox now travel with force plates that measure ground- interaction forces. The portable devices are set up in the tunnels of MLB stadiums so that hitters can measure their force and balance. They look something like large digital scales and, like TrackMan, were first popularized in golf. Betts had never thought about using the ground.

“He’d just been hitting away his whole life and didn’t think much of it,” Hyers says.

“Other guys have good years and they won’t make the change. [Betts] was like, ‘Dude, I have to figure this out. I don’t have it figured out.’ ” — J.D. Martinez

Like Turner in Los Angeles, Martinez is something of a Johnny Appleswing, a willing teacher and a clubhouse peer who reinforces Hyers’s philosophy. Among the 260 MLB hitters with at least 150 batted balls in both 2017 and 2018, Betts, center fielder Jackie Bradley Jr., and shortstop Xander Bogaerts ranked fourth, eighth, and 11th in year-to-year increase in hard-hit rate (the percentage of balls hit 95 mph or faster). Betts, Bogaerts, and Bradley also ranked first, second, and 30th, respectively, in year-to-year increase in barrels (a Statcast term for “balls someone hit the snot out of”) per batted ball.

“I have to give a big credit to J.D. Martinez,” Hyers says. “When you have a superstar and you have a guy who’s already proven it, to come in and back what you’re saying and have those conversations one-on-one with players. … J.D. was huge on saying, ‘Hey Mookie, you can do this part.’ And they had their own private conversations that helped a ton also.”

Now Betts has become a hitting tastemaker, too, inspiring rivals to reject the round knob at a time when some bat companies, like golf-club designers, have begun to use swing sensors to determine the perfect fit for each player in length, weight, and grip. “You start seeing that [Betts] has an influence within the league,” says Axe Bat’s Trevor Stocking. “[George] Springer asked him for a bat, and now all of a sudden Springer’s swinging Mookie’s bat, so now Springer is giving the bat to another player, and [it’s] kind of this wheel … where they’re passing on the equipment.”

They’re also passing on the swing, and hitters are getting off the ground younger. Byrd bought into fly balls at the tail end of his career. Turner saw the light just prior to turning 30. Martinez and Betts made the change in their primes. Now they’re modeling good batter behavior for impressionable minor leaguers, including one of Boston’s top prospects. Third baseman Bobby Dalbec, a 23-year-old fourth-rounder from 2016 who ascended to Double-A in 2018, tied for the lead among all minor leaguers with 67 combined home runs (32) and doubles (35). For years, players talked about “selling out for power,” sacrificing contact to take big swings. Dalbec turns that expression around. “I think it’s a waste of an at-bat for me to sell out for contact, hit a ground ball early in the count,” he says.

Dalbec wants to hit the ball hard and far, but he also wants to be smart about it. “Being on the right attack angle [and] trying to match the plane to the pitch is something I really want to get better at,” he says. “It’s definitely nice to be able to watch guys in my organization at the major-league level do that.” Although he notes that he’s nowhere near Martinez’s level, he’d like to learn from hitters like him and Betts. “If they’d be willing to or if they had time, I’d pick their brains all day about it,” he says. Fortunately for him, Martinez always has time to talk hitting.

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