http://www.nfl.com/static/content/public/image/rulebook/pdfs/15_Rule12_Player_Conduct.pdf
Article 9 It is a foul if a player initiates
unnecessary contact against a player who is in a defenseless
posture.
(a) Players in a defenseless posture are:
(1) A player in the act of or just after throwing a pass;
(2) A receiver attempting to catch a pass; or who has completed a catch and has not had time to
protect himself or has not clearly become a runner. If the receiver/runner is capable of avoiding or
warding off the impending contact of an opponent, he is no longer a defenseless player;
(3) A runner already in the grasp of a tackler and whose forward progress has been stopped;
(4) A kickoff or punt returner attempting to field a kick in the air;
(5) A player on the ground at the end of a play;
(
6) A kicker/punter during the kick or during the return;
(7) A quarterback at any time after a change of possession, and
(8) A player who receives a “blindside” block when the blocker is moving toward his own endline and
approaches the opponent from behind or from the side.
(b) Prohibited contact against a player who is in a defenseless posture is:
(1) Forcibly hitting the defenseless player’s head or neck area with the helmet, facemask, forearm, or
shoulder, regardless of whether the defensive player also uses his arms to tackle the defenseless
player by encircling or grasping him; and
(2) Lowering the head and making forcible contact with the top/crown or forehead/”hairline” parts of
the helmet against any part of the defenseless player’s body.