reverse crowning
The act of fully inserting one's cranium into a women's vagina and ideally crawling into her womb. It is the reverse of the birthing process, specifically crowning. It is advisable to cover one's head in a balloon of some sort in order to maximize pleasure.
Guy: Dude, I want to finger that chick.
Experienced Friend: No man, you gotta try reverse crowning.
Experienced Friend: No man, you gotta try reverse crowning.