This is just an automatic check for the defense, that whenever they see a particular formation, they will check out of whatever was called and into the preferred coverage/front to best match-up with a given situation. Again, this is SIMPLE to do and ensures your guys are never caught with their pants down by being out-formationed.
In the situation below, we had the tendencies of this offense down to whenever they went to this Power-I look, they only hit 2 particular areas of attack. Because of this, we felt that the BEST defense in our arsenal against this set was "6 Under Will Line", which is just a strong side loaded zone (6), with an Under front and putting the bubble LB on the line....this is essentially a 6-1 defense.
So, regardless of whatever we actually called in the huddle, once the MLB/WLB recognized this formation, they immediately checked out into our preferred call. All this took was 10 minutes on Monday, recognizing the backfield set, and we were good for the entire week of game prep. We would present a couple of different formations the team would run (2x2,3x1,1x2), and the minute we lined up in double-tight-power-I...the *** hit the fan and we had the kids going bananas over their alert.
Base call "3" ----- checks into "6 Under Will Line" when confronted with Power-I (formation check)
Talking with the coaches from the other team later in that season, they mentioned they didn't know what was happening to them because their Power-I attack had previously been unstoppable.