A very common misunderstanding about serve technique is that the wrist must "snap" through contact. The reality is that serve power is released through pronation of the hand and forearm, and if you snap your wrist, you completely destroy the effect of pronation.

If you look at the two pro servers on the right, you will see that their wrists are perfectly inline with their forearms. Their arms and rackets form a line that is extended forward into the court. This results from turning the hand and forearm outward as a unit. The server on the left has snapped his wrist down, like the motion of bouncing a basketball. This will break the chain of pronation and will severely limit the pace you can generate on your serve.

The idea that the wrist "snaps" through the ball is one of the most predominant (and most damaging) myths still embedded in tennis teaching. To really see how the hand and arm pronate together as a unit, check this out:

What Pronation Should Look Like