Usually refers to a situation where a male is "doing" a female from behind at the top of a flight of stairs. Just before ejaculating, he pushes his partner's arms out from under her, and rides her down the stairs.
A good way to get out of a bad relationship.
Man, I was doing Julie at the top of the stairs last night, and I suddenly decided to snowmobile her. It was pretty sweet.
Dude, I think the cops are at the door.
Shit. I think I might've killed her.