Video of the Shazier hit

[BigSeph]

September 2013-

Quote:NEW RULE: A ban on a ball carrier initiating contact with the crown of his helmet in the open field or by a defender while making a tackle.

What the rule changes: A 15-yard penalty will be called if a runner or a tackler initiates forcible contact by delivering a blow with the top/crown of his helmet against an opponent when both players clearly are outside the tackle box (an area extending from tackle-to-tackle and from 3 yards beyond the line of scrimmage to the offensive team's end line). Incidental contact by the helmet of a runner or a tackler against an opponent would not be deemed a foul.

http://www.nfl.com/news/story/0ap1000000238662/article/new-nfl-rules-crownofhelmet-change-to-help-runner-defender

Seems pretty clear.