New York, NY —

If you ride the number 6 train this afternoon, don't be alarmed if you see some passengers in their underwear.

REPORTER: The No Pants Subway Ride has been a yearly tradition since 2002, when seven members of the comedy group Improv Everywhere dropped trou on the Lexington Avenue Local. More than 150 people took part last year.

Group founder Charlie Todd says the event starts slowly, with one person boarding the train in boxers and briefs.

TODD: It's subtle at first, and it's hard for people to explain what could possibly be happening. By the end of the ride, when there's 20, 30, 40 people in their underwear, it becomes something that's obviously meant to be funny.

REPORTER: Not that everyone gets the joke. During last year's No Pants ride, a police officer stopped a train at 59th Street, ordered all the passengers off, and handcuffed eight of the pranksters on the platform. A judge later threw out the disorderly conduct charges, noting that it's not illegal to wear underwear in public.