Meant to refer to a girl-on-top position in which the woman is "riding" her man.
Jamie's friends all told her to ride 'em cowgirl when they heard that she had tried a new sexual position.
A woman is riding her man bareback, reverse cowgirl. When the woman feels the man start to finish, she reaches down between her legs and inserts two or more fingers in hookish fashion into the man's pooper and proceeds to shout "ride'em cowgirl!" while flailing about in the manner of a cowgirl for the duration of the mans post climax erection.
"He finished so fast, and I hadn't gotten mine yet, so I had to pull a ride'em cowgirl on that chump. He screamed like a stuck pig!"