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. 2

As we saw in Part 1, assigning every needed defender to a single gap can stress a defense.  One way to rectify the problem is to change the math by assigning one or more players to simultaneously cover two gaps.

You can baby-step your way into this approach.   Using the 4-4 again as an example, here’s a look at how 4-4 guru Bud Foster shifted his eight man front from the 90’s (which looked much like our example front in Part 1) into a variation on the “TNT” or “Eagle” look he called the “Tuff” front, which will look familiar to defenses NFL fans saw in the 80s.  Here’s how Foster drew it up for the Hokies:


Virginia Tech "Tuff" front; click to enlarge.

To achieve this front, the down linemen shift to the strong side, while a hybrid LB/DB called a “whip” takes the vacated weak-side end’s spot and a linebacker steps up to the line.  The result is a de facto attack-oriented 6-2 defense.  If you look in front of the “backer” position (roughly equivalent to a 4-3 SLB and labeled with a ‘B’), the defensive tackle (‘T’), nose tackle (‘N’) and right-side defensive end (‘E’) you’ll notice they have angled brackets drawn in front of them.  These represent gap assignments.  The backer, DT, and DE have single gap assignments—the C and two B gaps, respectively.  (Though it’s not shown here, the DT and DE are actually in shaded 3-techniques to help them hit their gaps.)

While these three are assigned single gaps, the nose tackle has responsibility for both A gaps, as the double bracket indicates.  This is the safest place to two-gap a defender: the center is handicapped by having to snap the ball before blocking, and teams rarely run to the A gaps because it leads them into the teeth of the defense.  Unlike a slashing single-gap defender, a two-gap defender essentially reverse blocks the man in front of him by exploding off the ball, gaining leverage, identifying the play, and then chucking the would-be blocker in order to chase down the ball carrier.

This front is great against running teams because it puts eight guys in the box while keeping the interior linebackers relatively free from contact.  Here, the rover (basically a strong safety) and mike key on the running backs nearest to them without worrying about blockers coming at them.  The scheme doesn’t, however, do as good a job with pass defense.  Walking the whip and backer down has pulled intermediate-level defenders out of pass defense, while pulling the rover down (a strong safety-type position) has made the secondary vulnerable.

One solution here is just to make more players two-gappers.  At the opposite end of the two-gap spectrum from the Tuff front are all-in defenses like the 3-4.  In this front, each down lineman is responsible for two gaps.  This gives the interior linebackers a great deal of freedom in chasing down plays, while also allowing for a balanced secondary.  Here’s a slide from a Baltimore Ravens presentation showing their base 3-4 defense.

Baltimore's base 3-4 in 2004; click to enlarge.
Note how each of the three d-linemen are lined head-up on an offensive player.  The nose tackle is in the same 0-technique from the Tuff front above, while the ends are in 4-techs over the offensive tackles.  All three players are responsible for stopping runs to their immediate left and right.  When done right, this relieves the linebackers of their immediate interior gap responsibilities.  In theory, the backers can react quicker to passes, either by dropping into coverage with greater purpose, or by tracking down flat runners and screens.

Like all things in football, however, there are tradeoffs.  The first is that it’s hard finding two-gap players.  “Truck and shuck” requires a controlled mentality, while beating offensive linemen head-up and then chasing down runners requires a freakish mix of size and athleticism. This can be a pretty apparent weakness when you see 4-3 teams in college switch to an odd front to help stop the option—the smaller DTs and DEs often get overwhelmed at the point of attack.  Second, even the best 3-4 linemen can’t consistently create a pass rush by themselves—they’re always outnumbered.  Blitzing is the obvious answer, though anytime you blitz from a 3-4, you’re essentially reverting back to a mixed or single-gap defense.  The 3-4 can make up for this by being so confusing, though it’s not a perfect solution.  Finally, in a pure 3-4 there are always two offensive linemen who are “uncovered,” i.e., they don’t have a d-lineman setting up shop in front of them.  If these guys aren’t required to help with double-teams, they essentially have a free shot on the linebackers.

In between these extremes are split-scheme defenses, which are often run as much to tailor schemes to personnel strength as they are to gain a chalkboard advantage.  Bill Belichick has been a master of this; in recent years, his squads have been combinations of 3-4 and 4-3 defenders.  To make the best of both worlds, he will split his defense down the middle and make the players on one side (say, the weak side) two-gappers, while the strong-side players are single-gap.  Perhaps the biggest weakness of this method is depth, since it’s pretty much impossible to have adequate reserves for two distinct styles of defense.

Pure two-gapping is a brute force solution to the numbers game of gap assignments, and it’s as old as the infancy of football, much the same as the pure single-gap approach. Today’s even-front teams generally prefer a more situational scheme that we’ll look at in the final post.