per the rule:
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: [. . .]
(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;
[. . .]
(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.
For the rule in its entirety: http://nflcommunications.com/2011/12...seless-player/