FOXBOROUGH — The problem with pinning down exactly how large the Patriots’ defensive playbook is one chapter will forever be missing.

It’s an amorphous collection of concepts and schemes gathered over decades of a football life. Bill Belichick carries it with him at all times. No one else has complete access to this section but him.

According to New England defenders, Belichick pulled more from it this season than ever before. He introduced and occasionally fused the chapter’s contents with parts of their established playbook to forge an unprecedented number of game-plan wrinkles.

“Since I’ve been here, we really haven’t done as much as what we’re doing right now with some packages," nine-year safety Devin McCourty says.

Yet strangely, as central as it was to holding future Hall of Fame quarterbacks Aaron Rodgers and Ben Roethlisberger each to 17 points and six other opponents to 13, no Patriot has seen Belichick thumb through this section or tote it around the team’s facility.

That’s because it exists only in his brain.

“He’s one of those guys that there’s no offensive scheme, defensive scheme, special teams scheme that he hasn’t seen or doesn’t have a full understanding of. But for us they’re new. So we have fun," McCourty says. “It’s like a new used car. It’s new to you.”

Teaching new tricks to the old dogs of New England’s defense is the secret sauce of their rise from a dreadful 2017 campaign.

Eagles quarterback Nick Foles passes against the Patriots defense during Philadelphia's 41-33 win in Super Bowl LII.AP

Eight of the Patriots’ current defensive starters are the same they trotted out for Super Bowl LII. A ninth — linebacker Dont’a Hightower – would have lined up last February had he been healthy. That defense ranked second-worst in the league by DVOA and got shredded by the Eagles for 41 points, a number far more reflective of its caliber than the regular-season rankings it held in yards or points allowed.

Remember: Nick Foles and Co. correctly predicted how New England how would defend them and exploited its plan ruthlessly. As Philadelphia center Jason Kelce later told Sports Illustrated: "We had a good feeling for how they were going to play everything. It felt like it was going to be one of those games where we go out there and dominate these dudes.”

This season, however, the Patriots were not prisoners to the communication lapses that dogged them a year ago and shrunk the playbook. Instead, crafty disguises and blitzes cultivated one of the league’s highest unblocked pressure rates. That led directly the NFL’s highest defensive pressure rate on third downs. And success in those areas drove the Pats’ ascension back to DVOA respectability and a No. 7 ranking in points allowed per game.

Now as Belichick and de facto defensive coordinator Brian Flores will attest, any success starts with players.

But specific to this season, growth began with the players’ ability to embody weekly game-plan visions and evolve into a defense that overcame physical shortcomings to go 5-1 against the NFL’s 20 best offenses; one that will carry the franchise’s flag at the dawn of another postseason in the twilight of the greatest run the modern NFL has ever seen.

Older, wiser, better

Duron Harmon needs no time to pinpoint when exactly New England took its first of many turns into uncharted schematic territory.

“It all started with Kansas City," he says.

Or how they did it.

“Disguise," he explains. "What we’re doing, where we’re lining up, where we gotta get to. We’re making things hard on the quarterback. Not just lining it up and giving (the coverage) to him.”

Harmon’s answers shouldn’t surprise. Thanks to a preceding Thursday night kickoff against the Colts, New England had three extra days and an additional practice to prepare for the then unbeaten Chiefs in mid-October. The resulting film of their Week 6 showdown, tape with defensive wrinkles, backs Harmon up.

But let’s step back two weeks.

Patriots linebacker Kyle Van Noy encourages the crowd during the second half of a regular-season home win.AP

Before New England could prove it still belonged atop the AFC, it needed to climb out of the conference’s basement at 1-2. So facing an undefeated Dolphins team, New England flexed its problem-solving skills through strong safety Patrick Chung and linebacker Kyle Van Noy. Chung dropped down to play middle linebacker for long periods of a 31-point throttling of Miami, then the 14-point win over Indianapolis. Simultaneously, Van Noy flipped from linebacker to defensive end.

Together, their smarts and versatility remedied the defense’s lack of second-level speed and soft edges against the run that led to embarrassing defeats at Jacksonville and Detroit. Chung and Van Noy combined for 27 tackles, an interception, one fumble recovery, a half-sack and two QB hits in the two wins. Better yet, they rewarded Belichick’s faith in their ability to adjust on the fly; belief that quickly spread to the entire unit.

“We’re just changing things up," Chung says. “We’re just doing different things game by game and try to hide our tendencies. You know, whatever coach calls, we play it.”

Days later, Belichick said it felt like Van Noy had been with New England for 10 years. The truth is it’s barely been two. Yet even Van Noy — who continued to ping-pong between linebacker and D-end — may not have the fullest game-plan plate.

He describes the volume of calls, checks and information McCourty is charged with mastering each week as “insane.” Told of the assessment midseason, McCourty humorously parried the compliment by saying it only seems so because Van Noy is not as smart as him. Jokes aside, describing this defense as collectively smarter compared to past years might actually be fair.

Patriots safety Patrick Chung and defensive lineman Trey Flowers sack Colts quarterback Andrew Luck during a 38-24 New England win on Oct. 4.Getty Images

Players who took at least 40 percent of New England’s defensive snaps this season average 5.3 years of league experience. The first and second-year Patriots in that group -- defensive tackle Lawrence Guy and cornerbacks Stephon Gilmore and Jason McCourty -- were all established veterans prior to arriving in Foxborough. Gilmore, the defense’s lone Pro Bowler and likely All-Pro, openly credits his career year to the coaching staff.

Whereas last season, New England plugged key holes with midseason signings (linebackers James Harrison and Eric Lee), late-summer trade acquisitions (linebacker Marquis Flowers) and players in their first or second year (linebacker Elandon Roberts and defensive linemen Deatrich Wise Jr. and Adam Butler). Added experience was the missing fertilizer for players’ understanding of the defensive system beyond their individual role; a must for Belichick’s brain to be put to maximum use.

By rotating player assignments — as Chung and Van Noy first demonstrated — New England can script a few defensive calls and have them present as dozens of different looks to the offense. And mid-game, through only one or two adjustments, the Patriots can unlock even more. Basically, Belichick can cause confusion on the cheap.

“It’s one of those deals when you put in a package or play or anything, you’ve got a whole group of guys that understand the scheme rather than just a specific position," defensive end Trey Flowers says. "Anytime you’re able to switch positions or have guys doing different things, you have to understand the whole scheme of what we’re trying to do so whatever we communicate and switch, we don’t get caught out of position.”

Flowers’ insight illustrates the clearest example of New England’s newly expanded playbook: the “amoeba defense" it first unleashed against Minnesota on Dec. 2 and has forced upon every opponent since. This package pre-dates any player on the current roster, a child of Belichick’s first years in New England, per The Athletic. Opponents completed passes on fewer than half their dropbacks against the Amoeba, averaged 4.2 yards per play and took two sacks.

It epitomized New England’s defensive leap.

But first, we’re on to Kansas City.

“Everything just looks so different”

A bald, 33-year-old secret weapon strolls into the safeties’ meeting room in mid-October. He’s got some advice on how to confound future league MVP Patrick Mahomes, and he’ll be back again many times over the season.

If the room were a bar, backup quarterback Brian Hoyer would be a regular. And that’s just how they would have it.

“He lets us know what the quarterback is looking for and how he’s going through his progressions," Harmon says. "What confuses him, what will make him hold the ball and give the rush another second to get home.”

Hoyer is an integral part of the three-day dialogue that ensues once Belichick and the defensive staff have introduced their latest game plan on Wednesdays. Conversations are shared between players and coaches, coaches and coaches and Hoyer and anyone who will listen. His advice usually pertains to what the Patriots should do pre-snap on passing downs when everyone, regardless of position, shares the same goal: disguise like hell.

“With his help, everything just looks so different now," Harmon says. “Nothing looks the same because we’ll start over here and then get all the way over there.”

At last Sunday night comes, and before Mahomes torches the Pats in the second half of a 43-40 shootout, they intercept him twice while keeping Kansas City out of the end zone through bluffed all-out pressures. New England consistently generates an unblocked rusher through these new blitz looks. McCourty drops into the box, blitzes as part of a six-man rush, then two or three Patriots bail after engaging an offensive lineman and match any route run over the middle.

This package protects the defense against short crossing routes, while the outside cornerbacks play off coverage in the event of an intermediate or deep pass. But primarily, it attacks Kansas City’s pass protection, which never gains a confident handle on it. Later, the same scheme forces the game’s only punt, a vital fourth-quarter surrender by the Chiefs that bought New England precious time.

Because while Mahomes eventually caps his night with a 75-yard touchdown strike to Tyreek Hill, the Pats score three straight times around Kansas City’s punt, then once again on a game-winning field goal. Without their new blitz bluff, the Patriots would instead have succumbed to the Chiefs’ firepower.

Three weeks later, Aaron Rodgers buses into Foxborough armed with film of the Patriots’ pressure wrinkles. Surely, he will decipher them. And Rodgers did — to a degree.

Except New England again unveils new disguised pressures that effectively neutralize Rodgers. The defense pressures him on an astonishing 43.8 percent of Green Bay’s snaps. Better yet for New England, the Packers average 0.1 more yards per dropback than they do on hand-offs to running backs.

Considering where the unit was nine months earlier, the performance registers as a minor defensive miracle versus one of the greatest to ever sling it.

“It takes a lot of communication and trust, but we trust each other,” Gilmore said post-game of the disguises. “As long as we trust each other, it allows everybody to play fast and believe in each other.”

The win grows trust between teammates, as well as coaches and players. Belichick is beginning to loosen his grip. Against certain quarterbacks, he will pair a particular pre-snap disguise with a particular coverage. Soon after the Green Bay game, defenders are allowed to rove as they please — provided their man and/or gap remains accounted for.

The risk of Belichick’s decision is obvious: His defense gets caught out of position, loses an angle on a ball carrier or surrenders an explosive play. The reward is less so: seeding further doubt in a quarterback’s mind and generating greater buy-in from his players.

Says Harmon: “You’re allowed to do what you always wanted to do: [lay good football, try to mess with the quarterback and dictate the game rather than let the quarterback dictate the game. And as a football player, when you have the freedom and ownership to be able to do that, it lets you know that not only the defensive coordinator trusts you, but the head coach trusts that you’ll be where you need to be.”

But it takes a while for New England to reach into its bag of tricks again. Trips to battle the Titans and Jets required as much game-plan deception as their bye week did. Then comes Kirk Cousins and the Vikings.

Poor Kirk Cousins. Belichick seems to sense that Cousins struggles when forced to move on from the initial reads in his progression. Take that away, and Minnesota’s offense melts.

So he pulls out the dirtiest trick of them all: the Amoeba defense.

Running this package requires careful coordination among New England’s linebackers and defensive backs, who roam pre-snap seemingly at random. The Pats typically deploy only one defensive lineman, with six or seven defensive backs behind him. The rush and blitz combinations are endless.

In 10 snaps against the Amoeba, Minnesota totals 36 yards and two first downs. The next week, the patrolling Patriots sack Dolphins quarterback Ryan Tannehill, and the Sunday after that victimize Roethlisberger for a seven-yard loss.

Around its glitzy blitzes, New England flexed some basic game-plan versatility, too. In New York, the Patriots open in a rare snap of traditional 3-4 defense. The message their heavy personnel group sent the offensively-challenged Jets is simple: You aren’t controlling the clock and rushing your way to an upset today.

The Patriots defense shows a 3-4 personnel grouping on the first snap of their 27-13 win over the Jets on Nov. 25.

Two plays later, Belichick unveils a new four-safety, three-cornerback package when the Jets are forced to throw on third-and-long.

The Patriots deploys seven defensive backs in a new defensive package seen in a road game against the Jets on Nov. 25.

These looks become more common in Week 15 against the Steelers.

In Pittsburgh, New England’s most common defense features six defensive backs. Belichick doesn’t care that rookie running back Jalyn Samuels will enjoy his most productive rushing day since high school. His game plan dictates Roethlisberger and the Steelers’ pair of Pro Bowl wideouts are the priority.

And ultimately, by limiting Antonio Brown and JuJu Smith-Schuster to a combined 89 receiving yards, a new season low, the Pats limit the league’s sixth-best offense by DVOA to 17 points. Crushing loss or not, defensively they do their job.

Through his defense’s intelligence and widening range of talents, Belichick has managed to keep his cards close the vest this season while making it known to opponents all game-plan options remain firmly on the table.

In other words, the NFL’s best coach has settled into the most dangerous position he can possibly be.

“Bingo”

Across the hall from the comfort of his locker room, Chung stands uncomfortably behind a podium in late December.

Arms outstretched, he leans on his platform and commands the room. Chung may not like the media spotlight, but you can hardly tell. His disguising continues.

Questions fly in about the Jets, the final speed bump on New England’s regular-season journey to earning a ninth consecutive playoff bye. New York is ready to turn the page to 2019, while the Patriots defense has long known its 2018 identity and been building on steady improvements. Even if Chung’s crew must carry the Patriots deep into the playoffs all by itself, Sam Darnold won’t be the one getting in their way.

The defense is too clear-eyed now, too capable, too confident.

(Spoiler alert: Darnold doesn’t. The rookie finishes with a 74.6 passer rating, New England forces three turnovers and allows a matching number of points. Van Noy scoops and scores in the third quarter. It was a Patriot party from start to finish.)

After a few minutes, I ask Chung whether he shares the opinion that the Patriots have done more schematically this season than at any point since his career started in 2009. Chung confirms taht he does.

So does he like it? The responsibility, the possibility, the complexity of it all?

“Yeah," Chung says before turning the tables on me. "Does it confuse you when you don’t know what’s going on?”

Stunned at the role reversal, I hesitate.

Chung recognizes the confusion he’s caused and pounces. He’s seen it twist the faces of Rodgers, Cousins and Roethlisberger before my own. Before I can muster a response, he smiles, points a knowing finger and takes a triumphant step toward the exit.

“Bingo.”