A fork of Rural Dictionary
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.