When a woman, while engaged in doggy-style sex, reaches between her legs to fondle a man's balls.
Also see: playing quarterback
Guy 1: Man, I love it when my girlfriend is "playing center".
Guy 2: What the hell does that mean?
Guy 1: Look it up on Urban Dictionary.
The act of a man receiving anal sex while bent over in doggy style or standing up. The term comes from the American Football position "Center", who's task is, amongst other things, to bend over in front of the usually larger quarter back and hike him the ball.
I think that dude Kelsey might like playing center, because he makes my gaydar go crazy every time he walks into the room.