4 NFL Rule Changes for 2013
If a player who is more than 3 yards downfield or outside of the tackle box hits another player with the crown of his helmet, he will receive a 15-yard penalty.
This rule is going to impact the games of several running backs. If both the offensive and defensive players lower their heads and hit with the crown of their helmets, both players will be penalized. Runners can still use the other parts of their helmets, such as the face masks and hairlines.
Many running backs are concerned going into the 2013 football season, because it's a natural reaction to lower your head to protect your face and the ball.