Imitation of reverse cowgirl. An indicator that a chick knows what's up.
A gesture often done in night club environments by girls to entice men and to simulate what will ensue in bed if you got with them.
"Check her out, she's twerkin' dat ass at Josh. She knows what's up"
"Oh man, you should have seen how she was grinding against his cock after that"
"We know that he's obviously getting some"