In a tight boxer's stance or Wing Chun form stance, rapidly punch forward with vertical fists. The basic concept is to perform a series of straight punches along the centerline. Forward fist should drop to allow for the other one to pass in a straight line (a lot of people do circular punching which is wrong). Structure should be maintained. Speed comes from being relaxed. Shoulders shouldn't move. Focus should be on the elbows and not the fist.