Andres ''Pooh'' Fernandez (he got his nickname because he often wore Winnie-the-Pooh regalia) has been a member of STOMP! for 16 years now. The performer and rehearsal director describes the percussion unit as ''kind of dancing, but you're stomping and you're moving rhythmically.'' Started in Great Britain about 22 years ago, the award-winning program changes its cast and some of the household implements it uses to make noise from time to time, but it stays wedded to its peculiar blend of precision and impulsiveness. ''We're 25 percent improv, 75 percent written. Within that 25 percent, it's always changing; we're always doing different things, playing different rhythms,'' says Fernandez. This year's additions: shopping carts and frogs (sounds ribbiting, right?).