It has some interesting discussion, particularly dealing with how the arm can be rapidly straightened and not hyper-extend.
However, for the elbow extension issue, when I look at this video of what really goes on in the serve, the highest racket head speed follows very rapid rotation of the upper arm (internal shoulder rotation) while the elbow remains mostly straight. I think that his explanation should use a similar high speed video to clearly get across what he means with the elbow. I not clear watching the video. I believe the rapid straightening of the elbow in relation to internal shoulder rotation needs to be explained with a high speed video of the serve such as this one.
Notice the arm achieves a straight-up position and then there's rapid (0.03 sec) upper arm axial
rotation seen best by looking at the elbow bones (ISR).
UPDATE - 2/7/2014
- to do stop action on Vimeo hold SHIFT KEY and use the ARROW KEYS. Vimeo now plays on my Samsung Galaxy S2 android phone.
Also, it's 2013, time that biomechanics researchers who know exactly what forearm pronation
is and exactly what internal shoulder rotation
is explain joint motions accurately and clearly in the proper terms and not use mind-muddling terms such as calling the overall arm motion 'pronation'.
Is there a single instructional serving video on the internet where the instructor uses the term "internal shoulder rotation" and also shows the upper arm rotation? The OP video clearly shows the upper rotation but does not use the term.
This is the closest that I have found so far but it is not a video. It uses the proper terms and refers to the research of Elliott and others.