Seems his defensive philosophy is as old as dirt and nothing not seen before in the NFL. Further proof that LeBeau's defense is no more dated than any other:
After spending a year selling roofing materials in the Bay Area, Carroll got his start in coaching in the 1974 season as a graduate assistant at his alma mater, the University of the Pacific. His big break came in 1977, when he secured a GA job at the University of Arkansas under new head coach Lou Holtz and defensive coordinator Monte Kiffin. Carroll later called that job “the best thing that ever happened” to him.
Most graduate assistants simply want to break into coaching; Carroll got that, but he also got something else: an ideology. “I am an example of a person who got zeroed into a philosophy early,” said Carroll. “Monte ran what is known in coaching circles as the 4-3 Under defense … That was the first time I started to get hold of something that had a philosophy to it. I started to grow with the defense.”
Taken literally, “4-3 Under” refers to a particular personnel grouping — four defensive linemen and three linebackers (hence “4-3”), and by extension four defensive backs — where the defensive linemen align away from the offense’s strong side (hence “Under”) while the strongside linebacker positions himself on the line, usually right across from the tight end. This is the same structure Kiffin ran while working for Tony Dungy and the Tampa Bay Buccaneers, and the “Under” front remains popular in the NFL today.
Carroll has never exclusively relied on this scheme;2 instead, he viewed Kiffin’s 4-3 Under less as a particular alignment and more as a belief system about football. “I have been running that same base defense since 1977 when I learned it from [Kiffin],” Carroll said at a coaching clinic. “I have used variations of this defense my entire career. I have stayed with its principles through all my years of coaching.” And the overarching principle is simple: Be aggressive.
The key to being aggressive is something called “one-gapping.” “There are really two [defensive] philosophies in pro football,” former Tampa Bay and Indianapolis head coach (and 4-3 Under guru) Dungy said at a lecture for coaches. “Do you want to be a one-gap team or a two-gap team?” Used this way, “gap” simply refers to the space between offensive linemen, and “run fits” is coach-speak for how a team handles those gaps.
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.”