The following is taken from the 2011 NFL Rule Book (pages 73-74) and defines players who are in a defenseless posture. The material is also covered in the 2011 League Policies for Players Manual, distributed to all players in training camp.
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.
Note: The provisions of (2) do not prohibit incidental contact by the mask or helmet in the course of a conventional tackle on an opponent.
Penalty: For unnecessary roughness: Loss of 15 yards. The player may be disqualified if the action is judged by the official(s) to be flagrant.