2 GAP 3-4-Groh
The 2-gap alignment leaves both offensive guards uncovered by defensive linemen. Instead each ILB covers the “bubble” left by the line. This kind of 3-4 requires a specific class of player. The defensive linemen have to be monsters, able to handle the lineman in front of them and control the gap to either side. Ideally, they’re disruptive enough that the guards have to help in many cases. The inside linebackers need to be big enough to take on a guard on every play if the linemen aren’t good enough to keep them clean.
It’s Parcells’ “Planet Theory” - there are only so many men on the planet big and athletic enough to play defensive lineman in the NFL. To some extent, that theory also applies to the linebackers in a 2-gap 3-4 scheme. Guys like Dat Nguyen won Parcells over, but he wasted no time in looking for guys like Bradie James and Bobby Carpenter and Kevin Burnett to replace smaller players like Dexter Coakley.
Parcells liked the 2-gap 3-4 for a number of different reasons. It’s design makes it more difficult for the offensive linemen to get an angle on his defenders. It makes it easier to drop eight men into coverage and prevent big plays. It makes it easier for a stud OLB to get an angle in pass rush and generate pressure with just four rushers. But it’s also more difficult to play in today’s NFL. Those planet-like defensive linemen are getting harder and harder to find.
1 GAP 3-4-Grantham
The attacking, aggressive style of play of the one gap 3-4 has stood the test of time better than the read-and-react style for much the same reason that the under 4-3 has. It allows players to attack the offense, specifically its ability to disguise the fourth (and fifth or sixth) pass rusher and the coverage behind. In fact, there are a lot of under front concepts in the Phillips 3-4.
Unlike the true 2-gap 3-4, there’s no definite “bubble” in this particular front. The strong side end slides down in the guard-tackle gap and the nose tackle slants to the weak side center-guard gap. The weak side end may or may not be head-up on the tackle, sometimes aligning in a 5-technique. Moving the defensive lineman by just 12 inches changes the philosophy entirely. It’s easy to see how the mindset of the defensive linemen changes. It’s clear that the two inside linebackers can be, if the linemen are disruptive at all, better protected from the blocks of interior linemen. You can see the lines of attack for a delayed ILB blitz or how each OLB might get a jump by shifting one defensive end to the outside of an offensive tackle.
The DL penetrates, and is charged with constant harrasment of the QB. The LBs are typically fast, and at least one of them will blitz on any given play. The reason for the near constant 1-LB blitz is to account for the fact that the outnumbered DL is also relatively undersized and only one-gapping. However, the adjustments work out well. The OL never knows who the blitzer will be, or where he will come from.
The school of thought for the Phillips 3-4 is the need to pressure against the QB to stop the pass threat, and this is done by varying who the "fourth rusher" (who is really a blitzer) is. Add another blitzer in here and there, and the speedy/aggressive Phillips system is a threat to QBs, and attempts to get turnovers by slashing the time that a QB has to make decisions.
