when you are about to hit a ball your mind should be focused only on the ball. not your body. shadow swing is when you can really focus on your body while swinging. so any player who are serious about improving the technique should be regularly shadow swinging for tweaking and refining. one of the things that you can work on is the balance. doing on a slippery surface is even better. like on a wood floor in a pair of slippery socks. a well balanced swing won't have much problem with slippery surface. I found this to be esp effective in finding a more balanced serving motion. if you use shadow swing, do you have any special method?