When performing the traditional Ugandan dance baksimba, women swing their hips in pendular motion, making the fringe tied around their waists swish like brushes in a car wash. Their feet step with the beat, but the rest of their body stays composed, their hands riding palms down on the air beside them. The effect is hypnotic. On a recent day at the Brooklyn Academy of Music, a group of teenage students were trying to get the baksimba right.