I’m a bit confused about the rule for 12 men on the field in American Football when it’s called on the defense. It seems like sometimes this penalty stops play and the offense does not get a chance to run a play. On the other hand, offenses often try to catch defenses with 12 players on the field in an attempt to get a free play out of it. So why is play sometimes stopped and other times not?
This is common with the defense, because as you state, the offense tries to catch the defense with 12 players on the field. There are two different scenarios to consider with 12 players on the field (which is not always on the defense):
- The play is not allowed to take place when a team has more than 11 players in its formation.
- The play is allowed to take place (ie, the ball is in play) when a team has more than 11 players on the field of play or the end zone.
Rule 5, Section 1, Article 1 of the 2016 NFL rulebook covers this (emphasis mine):
The game is played by two teams of 11 players each.
When play is stopped:
If Team A has more than 11 players in its formation for more than
three seconds, or if Team B has more than 11 players in its formation
and the snap is imminent, it is a foul. … In these instances, game
officials shall blow their whistles immediately and not allow the snap
or kick to occur.
When play is not stopped:
If a team has more than 11 players on the field of play or the end
zone when a snap, free kick, or fair-catch kick is made, the ball is
in play, and it is a foul.