If you've never heard of this glorious — or gross, depending on how you feel about being next to someone not wearing pants on public transportation — event, it is explained thus on the Improv Everywhere website: "Random passengers board a subway car at separate stops in the middle of winter without pants. The participants behave as if they do not know each other, and they all wear winter coats, hats, scarves, and gloves. The only unusual thing is their lack of pants."
Most of the participants are guys, and they're usually wearing boxer shorts. While they're probably pretty cold (although we did get up to a downright balmy 40 degrees in New York City today), they're not showing much more than if they were wearing shorts.