Any variety of wild, crazy, and retarded antics you can accomplish by hanging off of the poles, overhead grab bars, and other structures inside of a moving subway car. Usually occurs late at night when the subway is basically empty, with a relatively large group of people, and preceded by copious amounts of drinking.
A: "Damn, Alex looks dizzy--he just did a double backflip off the grab bar then spun around the center pole upside down like 15 times."
B: "Dude, I think he's gonna puke. Subway gymnastics are dangerous."