When a man is having sex with a women (while she faces out a window), he subtly pulls out and has a friend and or stranger switch place. The initial partner then goes outside the window to her surprise.
It don't have to be up the ass. After you spiyt on her back to simulate cumming, she turns around and gets a semen blast in the face. Where did that come from?
The chick wouldn't take facials or let me cum in her mouth so I pulled the old Whodini.