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:
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:
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.