I have been looking at the frame or two between the big L and impact. With that slow 30 fps frame rate and the server's high speed serve, I can't see the ISR very well. Also, I'm not familiar with that camera view. It looks a little suspect for going more in a forward direction and not fully using the rotation of ISR that requires a changing angle between the racket and forearm. ?
Big L and ISR,
http://www.hi-techtennis.com/serve/nalbandian_l.php
For a serve of that quality, high speed video in necessary for analysis. Look at the angles and ISR. How does the OP server compare to this high level serve?
https://vimeo.com/27528347
For stop action on Vimeo, hold down the SHIFT KEY and use the forward and backward ARROW KEYS. When I do this my computer jumps to the next video due to a glitch but then it works to single frame. If that jump-to-the-wrong video happens to anyone else, please let me know. Best to download and view with Quicktime or Kinovea for excellent single frame.
You might get some more useful results from a slow 30 fps camera by viewing from behind and looking directly along the ball's trajectory. You would have to repeat many serves of the same type and see if you can capture enough frames with useful forearm-racket angles to form a picture of what is happening. Video in direct, bright sunlight and with a wide angle zoom setting to get small motion blur. Sun on the server's back might make features of his arm show shadows and, therefore, be more trackable. Markers on the upper arm, tape on skin or a wrist band on the upper arm with a logo all can be used as markers to better see how the upper arm rotates.
