During sex and one partner is on top of the other (usually works better when woman is on top) and instead of bouncing she just strides your groin. It is usually when she's really into it and she's about to climax. There is less of an in and out motion and just a basic stride.
I think that hottie from Wrightwood was really into it, because she was groin striding the shit out of me last night. I have chafing scars on my hips! Ahh the good ole groin stride strikes again.