RULES
Targeting and Initiating Contact With the Crown of the Helmet (Rule 9-1-3)
No player shall target and initiate contact against an opponent with the crown (top) of his helmet. When in question, it is a foul.
Targeting and Initiating Contact to Head or Neck Area of a Defenseless Player (Rule 9-1-4)
No player shall target and initiate contact to the head or neck area of a defenseless opponent with the helmet, forearm, fist, elbow or shoulder. When in question, it is a foul. (Rule 2-27-14)
Note: Beginning in 2013, ejection from the game is a part of the penalty for violation of both Rule 9-1-3 and Rule 9-1-4.
KEY ELEMENTS
Target—to take aim at an opponent for purposes of attacking with an apparent intent that goes beyond making a legal tackle or a legal block or playing the ball.
Crown of the Helmet—the top portion of the helmet.
Contact to the head or neck area—not only with the helmet, but also with the forearm, fist, elbow, or shoulder—these can all lead to a foul.
Look at the last definition.