This is Jake Mailhot’s first post as part of his May Residency at FanGraphs. A lifelong Mariners fan, Jake now lives in Bellingham, Washington, just a little too far away from Seattle to make it to games regularly, which is sometimes for the best. He is a staff editor at Mariners blog Lookout Landing. He can be found on Twitter at @jakemailhot.

There has been no shortage of remarkable relief performances during the first month of the season. Jordan Hicks and Tayron Guerrero are playing a game of one-upmanship with their fastballs. Josh Hader is striking out basically everyone he faces. Adam Ottavino resurrected his career in an abandoned storefront. But the most impressive performance of all might be what Edwin Díaz accomplished in April.

The ninth batter Díaz faced this season was also the first to actually put the ball in play — he’d struck out the first eight. Seven appearances into the year, he finally gave up his first hit, a single to Jed Lowrie. He gave up just one other hit the entire month. Among pitchers who’ve thrown 10 or more innings, possesses the fourth-highest swinging-strike rate and has produced the lowest overall contact rate when batters actually swing. If you prefer more traditional accolades, he’s also leading the majors in saves. His performance earned him the April AL Reliever of the Month Award. Any way you slice it, Díaz has been pretty great so far.

Díaz has shown flashes of dominance like this before — his 2016 rookie campaign was good for 1.9 WAR on the back of a 2.04 FIP — but he’s always been a little too erratic for his own good. Some of his success in April came despite the inherent chaos of slinging a projectile at 98 mph. He’s already walked nine batters and hit three more, and he’s given up a pair of home runs in May already. A quick look at his plate-discipline stats reveals that Díaz is throwing in the strike zone at the lowest rate of his career, around six and a half points lower than last season. And he isn’t really inducing any more swings on those pitches out of the zone — in fact, batters are swinging far less often at his pitches overall. But again, when batters do swing, they just cannot make consistent contact. Díaz’s contact rate of 55.2% is better than Aroldis Chapman’s, Josh Hader’s, and everyone else’s.

So what has made Díaz so effective this year when he does find the zone?

I’d like to suggest that he’s finally learning how to pitch rather than just throw. Blessed with elite velocity, he spent the early part of his career simply trying to blow away hitters. When Isabelle Minasian interviewed Díaz’s minor-league coaches for La Vida Baseball last year, they revealed the bravado behind this approach.

Early in the 2015 season, the coaches of the Seattle Mariners’ High-A affiliate, the Bakersfield Blaze, assigned their starting pitchers in-game homework. Each one was given a sheet with boxes for each opposing batter, and instructed to watch the at-bats and fill in a game plan for each player. Most players filled the chart out the traditional way. But when pitching coach Andrew Lorraine looked at 21-year-old Edwin Díaz’s chart, he saw something entirely different. Rather than highlighting weaknesses for pitches high and inside, or propensities for swinging at anything in the bottom half of the strike zone, Díaz had written things like, “This guy can’t hit me,” “He can’t touch me,” and “I’m going to throw it right by him.”

But simply trying to overpower batters can only take a pitcher so far. When his mechanics break down, as Díaz’s were wont to do when he struggled last year, no amount of velocity will help him if he can’t find the strike zone. So Díaz spent this offseason dissecting the details of his delivery, trying to understand how it all works. It’s all in an effort to maintain consistent mechanics.

Here are his release points in 2017:

And here they are through the end of April, 2018:

So far, his release point looks a little more clustered, a good sign for his consistency.

Being able to readjust his mechanics on the fly is a big part of Diaz’s maturation as a pitcher. But understanding his arsenal and how batters react to his pitches is another aspect of his growth. During spring training, he told Mariners beat writer Ryan Divish that he wants to “think like an old guy on the mound. I want to be able to refocus when I need to.” Maybe “thinking like an old guy” also means having a plan of attack for his pitches, rather than simply trying to overpower batters. If we look at his fastball heat maps, I think we can see that plan beginning to form.

Again, 2017:

And through the end of April this year:

His fastball has a ton of arm-side run, breaking in to right-handed batters and away from left-handed batters. Last year, Diaz threw plenty of fastballs in the zone, but he wasn’t maximizing the pitch’s natural movement. When facing same-handed batters, he should be throwing his fastball inside so that, if they make contact, it’s in on their hands. And when he’s facing opposite-handed batters, locating on the outer half of the zone is more important to avoid leaking out over the heart of the plate. That’s exactly how he’s been using his fastball this season: in on righties and away from lefties.

These individual pieces of Díaz’s changing approach to pitching are interesting on their own, but they become much more interesting when they’re considered together. Pitch-tunneling is not a new concept, but our ability to understand and quantify it has grown by leaps and bounds in the past few years. If you’re unfamiliar with pitch-tunneling, it’s a fairly easy concept to grasp. The argument goes that a pitcher is far more effective if he can mask the movement of his pitches as long as possible, reducing the time batters have to recognize the pitch type. We also know that pitch-tunneling is linked to a repeatable delivery and pitch location.

For this piece, the specific measurement I’m working with is Pre-Tunnel Maximum Distance (PreMax), which is defined as, “the distance between back-to-back pitches at the decision-making point.” This is the perceived difference in distance between two pitches at the point when a batter needs to make a decision to swing or not. The league average PreMax across all pitch pairs was 1.54 inches; a smaller PreMax value is better. Because of the difference in perspective, pitch pairs are further divided by batter-handedness. Below is a table listing Díaz’s PreMax values over the last three years for his two primary pitch pairs.

Edwin Díaz PreMax Values (2016-2018) Batter Hand Pitch Pair PreMax – 2018 PreMax – 2017 PreMax – 2016 RHB Fastball-Slider 1.29 1.53 1.86 LHB Fastball-Slider 1.31 1.38 1.36 RHB Fastball-Fastball 1.20 1.33 1.52 LHB Fastball-Fastball 1.24 1.36 1.33

We can attribute much of the improvement in Diaz’s PreMax values this year to more consistent release points, but for his fastball-slider pitch pair, the location of his fastball also plays a significant part in deceiving batters. This 3D visualization from Baseball Savant helps us get a better sense of what this looks like to a right-handed batter.

By locating his fastball inside to right-handed batters more often, he’s creating a better pitch tunnel with his slider. If he throws his fastball on the outer half of the plate and then follows up with a slider down and away, the natural movement of both pitches wouldn’t create as tight of a pitch tunnel. I don’t know if pitch tunnels are at the forefront of Díaz’s mind when he’s pitching. They’re probably not. But by focusing his plan of attack, he’s benefiting from increased deception on top of his incredible velocity.

Díaz’s raw stuff hasn’t really changed all that much, but his growth and maturation as a pitcher has helped him increase his effectiveness, particularly when pitching in the zone. With greater awareness of his mechanics and a better approach to maximize his pitch arsenal, he’s making the right adjustments to become an even better pitcher. But it’s all part of a process. His appearance on Wednesday night was the first time he failed to record a strikeout this year. He’ll probably blow a save or two during the season. It’s possible his new approach is one of the reasons behind his low zone percentage. Still, the fact that he’s open to making these adjustments speaks to his willingness to grow. And just in case you’re wondering, that fire and bravado is still there: “I trust in my stuff. I say, ‘I’m better than them.’ I just make pitches. If I’m making pitches, I’ll be fine. That’s what I did yesterday.”