The need to choose between one-gapping and two-gapping arises, according to Dungy, “because of simple math: You have eight gaps to fill and you only have seven front players.” A one-gap technique is much like it sounds: Each defender is responsible for attacking and controlling his assigned gap. By contrast, a two-gapping defender is responsible for the gaps on either side of the lineman across from him. How? He controls both gaps by controlling the blocker in between. A one-gapper attacks gaps, while a two-gapper attacks people.
Carroll, like Dungy, prefers not to two-gap. The problem isn’t the theory — a potential two-for-one where a single defender can clog up two running lanes is a great deal for the defense — but rather that two-gapping too often results in hesitant defensive linemen who try to read and react and thus fail to disrupt the offense.
“When you put a defensive lineman in a gap and tell him he has to control the gap, he can play very aggressively,” Carroll said at a coaching clinic.
“We want to be an attacking, aggressive football team,” he said at another clinic. “We don’t want to sit and read the play like you often have to with two-gap principles of play.”
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Of course, that doesn’t address Dungy’s math problem: the unaccounted-for eighth gap. The one-gap answer is to use a safety to fill the void. “We assign everyone a gap and use an eighth man out of the secondary to cover the eighth gap,” Dungy explained at the lecture. “Our system has not changed in about 20 years.”
If you watch Carroll’s Seattle team, you’ll see all of this at work: aggressive one-gap techniques, safeties rocking down to help against the run, and that classic 4-3 Under front. But that’s not all you’ll see.