Every player has to stay in his position and may not violate the positioning rules until the opponent's server has touched the ball (of course, assuming that the referee has blowed his whistle).
Every setter in every game I've attended on FIVB level leaves his position earlier.
I think this is due to the velocity of the serve, so the setter has enough time to get to the net.
What are the rules in this "violation"? In particular, I am asking for
- How many seconds before serving the setter may leave his position?
- Is this standardese or FIVB-specific?
- Does this only apply to the setter?