Usually performed while under the influence of alcohol. While when dancing in a large group of people you stick your hand into the air with it cupped at a grabbing angle while laughing obnoxiously. You then reach down and grab the crotch of some hot unsuspecting babe. This behavior has a tendency to piss off some ladies boyfriends and to get you kicked out of clubs and bars. I speak from experience.
As Bellows drank beer among all of the hot female dancers, he decided then and there to do the crotch dance.