Tag Archives: Defensive schemes

What is a “Gap-Sound” Defense? Pt. 3

For the first two parts of this series, we looked only at static gap defenses that alternated between various amounts of single- and two-gap assignments.  The final variation is a flexible gap defense where the assignments are situational.  We’ll use Jimmy Johnson’s 4-3 under (called “Eagle” in his terminology) from his days with the Miami Hurricanes, though its tenets are pretty much the hallmark of all advanced 4-3 defenses.

The idea behind flexible gap assignments is pretty simple.  First, there’s no point defending a gap that isn’t threatened.  Second, it’s more effective to cover gaps in a way that has the defense flowing to the ball carrier.

Unlike vaguer pursuit rules such as force and contain, flexible assignments require players to determine their specific gap assignment after the snap.  We see this in action with the Miami linebackers, whose assignments are determined by which way the ball is being run.  Here’s a strong-side run being defended:

Miami's base Eagle versus a strong-side lead; click to enlarge.

I’ve identified the Will, Mike, and Sam to help keep things clear.  In this situation, the Sam reads run and flies upfield to defend the alley.  The Mike defends the B-gap, while the Will streaks across the formation in pursuit of sweeps and option plays.  Notice that having the Will ‘backer cut across leaves the weak-side A-gap without a designated man.  This doesn’t pose much of a problem, mainly because the Will is heading in the direction of the gap to begin with, but also because Johnson’s system was built around having extremely fast players at the linebacker spots who could recover from bad reads.  Technically, the backside linemen don’t even have gaps—they’re simply assigned to pursue the run.

Even though the front is asymmetrical, the gap assignments are roughly flipped for a weak-side run:

Hurricanes D versus a weak lead; click to enlarge.

Now the Will is at the point of attack, while the Mike fills in behind him.  Again there are uncovered gaps, but they’re either away from the play or indirectly covered by virtue of a pursuing linebacker.

Of course, a defense like this can be vulnerable to trap, counter, and power plays that change gaps or the apparent direction of a run.  The solution harkens back to the fact that no defenders (especially the linebackers) ignore on-field action when they make run fits.  Linebackers and safeties in general read the backs, the outside backers and strong safeties read tight end releases, and inside backers watch the guards and center for clues about a play’s actual intent.  Johnson did it so well that not only did this scheme nearly render the option obsolete, but it dominated the professional ranks when he moved to the NFL.

Part 1: http://www.secondlevelfootball.com/2013/04/13/what-is-a-gap-sound-defense-pt-1/

Part 2: http://www.secondlevelfootball.com/2013/05/02/what-is-a-gap-sound-defense-pt-2/


What is a “Gap-Sound” Defense? Pt. 1

Pretty early in football’s evolution, coaches realized that even the simplest, most repetitive run game could be devastating if a defense wasn’t “gap-sound.”  Gaps—the spaces between offensive players—are the high ground of football: whoever controls them usually controls the game. If a defense leaves a gap open at the snap, any decent tailback will see the vacancy and run for a gain. Even if the defense initially covers a gap, leaving too soon invites cutbacks, counters, and misdirection plays.

Defenders are now assigned gaps to cover. These assignments can change based on play-call and what the offense does. In a 4-4 defense, the gap assignments might look like this:

A simple single-gap 4-4 front versus an Ace set; notice how every defender covers an initial gap.

Notice how each gap (or hole, if you’re looking at it from the offense’s perspective) has an initial defender, making this a purely one- or single-gap defense. In the base scheme above, defenders are aligned close to the gap they’re supposed to control: the linemen are shaded to make it easier to defeat blocks, while the linebackers are standing directly over their gaps so that they can rush in and make a play with minimal impedance. Being positioned over your assigned gap also makes reading and reacting to plays easier. If a ball-carrier runs into one of these gaps, the assigned defender should make the tackle for a loss or minimal gain.

How the defense defends these gaps depends on a lot of factors. Players on the line of scrimmage generally either attack the nearest blocker in order to gain leverage, or try to beat the blocker off the snap by exploding into the gap itself. Players off the line of scrimmage (the four linebackers in this case) might try to meet any blockers or runners in the hole, creating a pile-up in the first case and notching a tackle in the second.

Knowing when and how to abandon an assigned gap is a critical skill. Defenders are taught how to recognize if their gap is threatened, and if no one is coming to vacate the gap and help in pursuit or to defend against cutbacks and misdirection plays. They do this not just by watching the running backs, but by watching where and how the linemen are blocking. Middle linebackers, for example, are often assigned to read the center and both guards, while also being aware of the backs. If one guard pulls while the center and second guard aggressively block down, for instance, the Mike is probably worrying about a Power play. On the other hand, if all three take bucket steps in the same direction as the back is running, then he might be more worried about a zone run.

An offense can scheme to take advantage of a defense’s gaps assignments. Isolations and pulls are common ways of complicating gap assignments, as are option plays. When it’s clear which gap (or gaps) is actually threatened (or threatening to appear), defenders away from the play switch to defending against the runner or lead blockers. Defenders with assignments like “force,” “spill,” and “contain” try to either corral the runner back to the strength of the defense or towards the sideline by aggressively meeting blockers and option men. “Fill,” “cutback,” and “insurance” assignments make clean-up tackles against lead plays or provide backup when the defense breaks down. (You’ll see “contain” used in this context, too.)

Defenses that become too predictable in their gap assignments can make themselves vulnerable. Offensive coordinators can recognize the scheme and either throw shifts and formations that force the defense into tough positions by scheme, or they can single out weak links on the offense to attack. In addition to simply changing formations and assignments to vary the defense, blitzes, shifts, and exchanges are great ways to keep offenses off-balance. Blitzes, especially from unexpected defenders, can confuse blockers and bait running backs into traps. Last second shifts of the front (or “stems”) force blockers on gap-scheme runs to pick new assignments. Exchanges, where defenders move after the snap to cover more distant gaps, are subtler ways of getting to an offense.

Probably the biggest disadvantage of the purely single-gap scheme is its deficit versus the passing game. In today’s game, it’s nearly impossible to run a single-gap 4-4—there are just not enough people in the secondary. Moving one guy back to give you two safeties while still having him directly responsible for a gap is doable but requires a special player. This defender has to be smart enough to not only play the pass while watching a gap, but have the physical tools to do both. Undersized safeties without a nose for tackling will get targeted by opposing offenses, while stiffer, slower run-stuffers are prone to giving up deep balls.

Even with an excellent player, though, advanced passing schemes can create tremendous problems for a pure single-gap front. An answer to this dilemma is to have players who are in some way responsible for two gaps, though as we’ll see in the next installment this isn’t a perfect solution.