Using any move will immediately ground you. The Up + B stall happens after the fight starts. After a mid-air Up+B wait until the last frame of the animation and do Up+B again. You will gain the upwards momentum of the move, but no attack will come out. This can be done with neutral B in mid-air, as well, but the only effect of this is to advance the RNG without wasting time (which is used a couple times in the run).