OK, which one?
Rule 2 says:
SECTION 20 HELMET CONTACT – ILLEGAL
ART. 1 . . . Illegal helmet contact is an act of initiating contact with the helmet
against an opponent. There are several types of illegal helmet contact:
a. Butt Blocking is an act by an offensive or defensive player who initiates
contact against an opponent who is not a ball carrier with the front of his
helmet.
b. Face Tackling is an act by a defensive player who initiates contact with a
ball carrier with the front of his helmet.
c. Spearing is an act by an offensive or defensive player who initiates contact
against any opponent with the top of his helmet.
S&I Says:
Butt Blocking in 1 and 2 and face tackling in 3 are both tactics which involve driving the face mask, frontal area or top of the helmet directly into an opponent in blocking or tackling respectively. both result in a foul for illegal helmet contact.