Playbooks
Under is the defensive-front variant of the 4-2-5 Big Nickel package, and the play list makes that obvious: Will Go Fire 3, Boundary CB Blitz, Hammer 0 Blast, Safety Blitz, Sam Blitz 2, and a full menu of coverage shells from Cover 4 Quarters to Tampa 2 to Cover 3 Match. Instead of a balanced even front, the line slides to an Under alignment with the nose tackle shaded to the strong side A gap and the defensive ends offset, which puts the RLE and RRE on different alignment tracks and gives the SUBLB and Sam free run lanes on the weak side. That front asymmetry is the whole point: pressure comes from a different gap than the offense expects based on the shell you show.
Personnel is Big Nickel 4-2-5, meaning four down linemen, two linebackers, three corners and two safeties with the slot corner (SLCB) replacing a third linebacker. That nickel body gives you real coverage speed against 11 personnel trips and bunch sets without sacrificing four-man rush integrity, and it lets you keep man coverage on slot receivers instead of asking a 78 speed backer to carry a route up the seam. The tradeoff is run fit: only two true linebackers behind an Under front means the SLCB and strong safety are part of your run support, which is why the SS Sam Crash Press and Mike Sam Crash Press plays exist in this set to reinforce the edge.
What makes Under valuable competitively is coverage disguise off a single front look. Cover 1 Hole, Cover 2 Invert, Cover 3 Sky WK, and Cover 4 Quarters all deploy from the same alignment, so an opponent reading your pre-snap shell has no reliable tell. Pair that with Free Fire and Cover 1 FS Fire for five man pressure that still carries a rat in the hole, and you can isolate the offense's third or fourth receiving option with your best matchup corner while the user takes the underneath window away.
Run this set against 11 personnel and 10 personnel spread looks where the offense has three or more receivers, since Big Nickel keeps your coverage speed on the field without inviting heavy run personnel
Use Cover 2 Invert and 2 Invert Hard Flat on third and six to nine: the inverted safeties squeeze the intermediate windows while the hard flat player takes away the checkdown
Pre-snap, shift the defensive line and pinch or spread the ends before the snap to change the Under front gaps, then blitz with Sam Blitz 2 so the pressure arrives from a lane the offense did not block
Always user the SUBLB or a safety on coverage calls, never the down linemen, because the two-linebacker structure means the middle of the field needs a human closing on drags and sit routes
Save Hammer 0 Blast and Free Fire for third and long or two-minute situations where the offense must drop back, and disguise them by showing the same alignment you used on your last Cover 4 Quarters call
Pair Cover 3 Match with a user over the slot to handle trips and bunch, since the match rules take away the standard Cover 3 curl flat beaters that punish the SLCB