Nan Cu is correct. During a serve, the heel of the hand leads first, with the thumb the lowest finger. It should be pointing towards the court behind your shoulder, and the edge of the racquet should be travelling vertically. At the moment of impact, the wrist should twist outwards so that the thumb is now pointing to the sky. That, technically, is the pronation. The momentum of the hand and arm will continue, however, and the palm of the hand will face away from the body and the bat comes downwards.
The twist is there to give a little added "snap" to the ball, increase power, and create spin. Probably the easiest player to watch for this is Pete Sampras, though truth be told, he is the best to watch for anything in a serve, as his is just about technically perfect.
NOTE: The views expressed in this post are not necessarily those of the author.