What it is

A defender crosses the line of scrimmage and makes contact with an offensive player or the ball before the snap.

The penalty

The penalty is 5 yards, and the down is replayed. The penalty is enforced from the line of scrimmage, and the play is stopped immediately.

More detail

If the defender crosses the line but nobody touches him and he gets back, there is no foul, which is why you will sometimes see a defender jump back at the last moment.

Animated play

Blue circles are the team with the ball. X marks the other team. The yellow oval is the ball.

1. The ball has not been snapped. Both teams are waiting.
2. A defender jumps across the line of scrimmage and touches an offensive player.
3. The officials stop the play. It is a five-yard penalty against the defense and the down is replayed.