Legs catch setup, pull baggy pants down
For this trick, you need to wear an extremely loose pair of pants without a belt. Start for a legs catch. At the moment of the catch, the catcher grabs the end of your trousers instead of your legs. As you swing back away from the catcher....he pulls them off revealing some bright underwear (which you have not forgotten to put on). Another variation of this trick involves wearing a VERY long pair of pants, which you have bunched up around your ankles. As the catcher pulls, he reveals their full length (but they stay on at the top).
You can find out more about this trick in following books: