A sexual practice, most likely between two men, in which the goal is for the one to insert his actual head into the anus of the other. This is accomplished by the receiver having his spincter muscle severed by a knife, so his ass cheeks more or less flap apart. The giver then lubes up his shaven head and goes to town. It is uncertain whether this practice is supposed to be (more) pleasurable for the giver, or the receiver.
Raoul had to go to the doctor's to have his butt cheeks sewn back together after he was finished helmeting.
Like the similar definition on this page, the act involves inserting one's head into the particular orifice. The original example that I've seen, and most others have also seen, is a video of a bald man putting his head (up to his ears) into a woman's vagina.
Did you see that video where that guy put his head in that chick's pussy? That's gotta be one seriously loose hole for helmeting.