Sometimes when playing someone who has a predictable serve (e.g. serves 99% short from the backhand side), I wait for the server to set up, and make a menacing step forward before he actually makes the serve. This move occurs before my opponent stroking the shuttle. A friend pointed out that one must hold still when an opponent is serving and that making a move results in an automatic point loss. However, I can't seem to find any reference to such a rule. Can anyone clarify whether it's illegal and point me to the corresponding rule? Thanks in advance!