Energy from the rotational component around the axis of the racket on the serve.
FYB has a very clear introductory description of pronation. Just after 3 minutes he describes rotational energy and its transfer into the ball.
For this question the body's source of the rotational energy is not relevant.
This issue involves the rotational component of racket head speed, it's axis and where the ball is contacted in relation to this axis. Let's say that the racket is rotating around an axis running through the center of the handle to the top of the frame. The racket strings have a large translational velocity component and a rotational velocity component that varies to each side of the rotational axis. This depends on which side and how far off the the axis the ball is contacted as the racket strings have a velocity either greater than or less than the larger translational component. That is, to one side of the axis the racket strings have a greater velocity and might produce a considerably different serve velocity.
Complicating things while being struck the ball may translate across the strings toward or away from the axis. Probably there is no significant translation on the flat serve but on the spin serves the ball might move closer or farther away from the axis while the racket is rotating.
If I could somehow get a good service motion do I want to hit a little to the right or left of the racket axis?
High Speed Video Set Up.
To study this issue close up high speed video of the racket face when the ball is struck would be useful. An optimal camera location would probably be behind the server with the camera high and viewing the racket face. Viewing also to the corner of the opposite service box would allow seeing the resulting serve trajectory. The camera has to be 12' high or so for my serve or 14' if mounted above the fence - a little inconvenient. Maybe looking through the net would also be OK but the ball trajectory is not that observable. Any video experience, other set ups? Or, some links to very close up, high speed video strikes would be useful.