What it is

Only one offensive player may move before the snap, and he must not be running toward the line of scrimmage when the ball is snapped. A second player moving, or a motion player running forward at the snap, is illegal motion.

The penalty

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

More detail

Officials throw a yellow flag when they see it. The other team may accept the penalty or decline it if the play turned out better for them.

Animated play

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

1. One offensive player is allowed to move before the snap, as long as he moves sideways or backward.
2. Here two players are moving at the same time, and they are still moving when the ball is snapped.
3. The official throws a flag: illegal motion, five yards, and the down is replayed.