Originally Posted by toly
Here is Chas Tennis post about “Definition & usage of Pronation & Supination.
IMO we can say that the wrist is in maximum extension position (or minimum flexion position) when it has angle -90°. Similarly, the wrist is in maximum flexion position (or minimum extension position) when the angle is +90°. Start point of motion can be arbitrary within the range from -90° to +90°. So, according to proper definitions of these motions I can use the term wrist flexion to describe the wrist unbending progression.
Quite a few times in the past 5-6 years I have indicated that flexion, extension, etc. can be applied to both position and to an action/movement. I have never said that the unbending that you talk about is not a flexion action. I completely agree that a motion from an extended position to a neutral position is a flexion action -- I have indicated this many times previously.
What I have said is that the term "wrist snap" is misleading. In my mind and in the minds of many other students of the game, it implies a an extreme wrist action where the wrist ends in a fully flexed position. To some it also implies that the wrist is in a flexed position at contact. Both are incorrect conclusions that derive from a misleading instruction.
The other thing that I do not agree with is that the wrist assumes a full wrist extension on the serve. For normal ROM, the wrist can be extended up to ~70 degrees. From my observation (of your images), the wrist is not extended to this degree for the serve. What I see is that the wrist is radially deviated with something less than full extension.