I recently played a doubles match where my opponent at the net stood with her foot touching the middle service box line and then moved when my partner was tossing the ball in the air.
This seemed like a player hindrance to us so I looked it up on Google. Here is what I found from the USTA web site:
The receiver’s partner may stand anywhere, even in the service box. Once the receiver’s partner takes a position, he/she should remain there until the ball is struck. If the receiver’s partner suddenly and abruptly moves away before the ball is struck by the server, then it could be deemed that the only reason they initially took such a position was to hinder the server, and that is not allowed.
Note: The receiver may change positions to receive serve at any time prior to the serve and may even stand in the service box. Waving the racquet, making noises or stomping the feet is not permitted by any player.
This should have resulted in a loss of a point. Next time I will know!
For more info on player caused hindrance go to: http://www.usta.com/Improve-Your-Game/Rules/Distractions-and-Interferences/Hindrances_caused_by_players/