But that's what we've always been instructed to do. If there are 12 on the field and in our judgement the "12th man" is not going to make it off before the snap we have a simple DB substitution foul.
That's not what we're told to do.
If the 12th defender is *trying* to get off the field and doesn't get there before the snap, but then does leave the field and the play only has 11 participants, it's a
live ball, illegal substitution foul. Flag it, let the play run and give the offense a "free play". 4-7-4 classifies this as a live ball foul.
If the defense has 12 in formation, the snap is imminent and NO player is trying to get off the field, then you kill it pre-snap for a dead-ball illegal substitution, rather than illegal participation, foul. This is a violation of 4-7-1 (replaced players must leave the field in three seconds) which is a dead ball foul.
There can be an argument if the player running off is still inside the hash marks at the snap (not just *almost* off the field) that it would graduate to IP since he would influence play by causing other players to go around them, and therefore should be killed pre-snap, but the argument is then has as to where is "middle of the field" vs "almost to the sideline" -- which will also be very situationally dependent.