The Two-Gap
However you term it, it truly is a unique defensive style that stands in stark contrast to almost the entirety of other NFL defensive schemes, and fewer have promoted it more successfully than Crennel.
His style is termed the two-gap 3-4″ defense. At its base, that means you have three lineman with their hands in the dirt, typically two defensive ends that are big and strong enough to take on offensive tackles and tall enough to deter passes by the quarterback in the pocket (think of Tyson Jacksons prototypical 64″, 296 lbs frame), and a humongous, bulky nose tackle that is built for getting low, absorbing punishment and administering immediate push against a hopelessly outsized center (think of Jerrell Powes prototypical 62″, 331 lbs frame). Then you have four linebackers: two passrushers on the edge (one who is a more one-dimensional pocket assaulter, and the other who is a swiss army knife of abilities), and two in the middle (one who is more of a line of scrimmage attacker, and the other who is more coverage-oriented). This earns Crennels defense the 3-4″ moniker, for those who didnt already know.
But roughly half the teams in the NFL play with a 3-4 arrangement. Its Crennels two-gap approach is what truly sets this defense apart from every other style in the NFL.
From the Bears cover 2, to the Eagles wide 9, to the Texans one-gap, to the Patriots hybrid, to countless other 4-3 and 3-4 teams, the name of the game is very simple for the defensive line: get upfield. The players might line-up at different points along the defensive line. They might be bigger (3-4 nose tackles), smaller (cover 2 passrushers), ends, tackles, or rushbackers. But they all want to disrupt the pocket and get upfield.
Not so with the two-gap.
Think of this as your typical offensive line:
RT RG C LG LT
(C) (B) (A) (A) (B) (C)
This is a comprehensive listing of the gaps in an offensive line. The A gap is between the center and guard, the B gap is between the guard and tackle. Most defenses in the NFL tell their defensive lineman to shoot a gap, which would either penetrate the pocket or at least disrupt the offensive lines blocking assignments. Some defenses give a defensive tackle the job of occupying blockers, or two-gapping, meaning they focus less on getting upfield, and focus more with minding two gaps and clogging the lanes, offensive lineman be damned.
This two-gap strategy asks every single one of its defensive lineman to do exactly that. It turns the traditional role of the defensive lineman from pocket invader to space occupying, gap eater. Theres a reason young bucks like Tyson Jackson and Dontari Poe take a long time to adjust to it, the techniques involved in this are completely different than the simple get up and go of more attack-oriented defensive lines. (By my count, only four other teams in the NFL share this defensive style: Ravens, Jets, Dolphins, and now the Colts.)
The complexity of all of this aside, there is one thing needed to make this work. One thing needed to turn this defensive style, unique as it is, into an offense-wrecking machine:
The defensive linemen must occupy offensive linemen.
Its that simple. Its the first domino for the entire scheme to make sense. If these defensive linemen can force offensive lineman to double-team them, it frees up the linebackers behind them and to their flanks to make plays. If Glenn Dorsey and Jackson play their roles properly, Tamba Hali and Justin Houston are battling tight ends and fullbacks en route to the QB, rather than massive, athletic offensive tackles. If Poe can demand maximum attention in the interior of the line, it allows Derrick Johnson to flow to the ball freely. It therefore cuts down on the amount of time secondaries have to cover receivers. It allows the safeties to clamp down open spaces faster. The key to it all is that the defensive linemen must absorb multiple offensive linemen every play.