Under

4-3 Under is the run-heavy shift of the 4-3 family, sliding the defensive line to the strong side and reducing the front so the nose tackle covers the strong-side guard while the weak-side end (RLE) aligns wide in a rush position. The tag list (NT, RLE, RRE, RDT) tells you exactly how the front is built: two interior gaps clogged by the nose and right tackle, with both ends set on opposite hashes so you get a natural speed rusher on one side and a bigger anchor end on the other. Compared to a standard 4-3 Over or a symmetrical even front, Under puts more mass over the tight end side, which is why it holds up so well against inside zone, duo, and gap runs to the closed side.

Personnel is straight Base 4-3: four down linemen, three linebackers, two corners, two safeties. That means you are asking your slot corner or third linebacker to cover a receiving threat, so Under is best deployed against 21, 12, and heavy 13 personnel offenses rather than trips or empty spread sets. The linebacker depth in this set is what makes the Cover 3 Match, Cover 3 Sky WK, and Tampa 2 calls viable: your MLB starts close enough to the line to fill on run keys but has the play art to carry the deep middle.

Where Under separates itself is the blitz menu. Mike Sam Crash Press, SS Sam Crash Press, WK Corner Blitz, Sam Blitz 2, Safety Blitz, Hammer 0 Blast, and the Fire 3 family (FS Fire 3, Will Go Fire 3) all launch from the same reduced front, so the offense sees identical pre-snap alignment whether you are running Cover 4 Quarters or a zero blitz. Pair that with the press variants (Cover 1 Hole Press, Cover 1 Robber Press, Cover 1 Contain Press, Cover 3 Seam Press) and you can disguise pressure and coverage out of one look all game.