I think it is legal. The ball has to leave the hand before you leave the blocks. Had similar thing is high school with one guy shooting underhand (granny-style) and would have all sorts of issues with guys leaving early. AFAIK, your feet can't leave the ground, and you can't purposefully try and draw the other team off, which the referee in my game said would be if you changed style between the first and second shot for example. But He was consistent so...probably okay.