Actually, the forearm has already slowed down prior to the image #1 in your post #137. As the racquet head moves upward from the scratch position, the server drives upward with the shoulder and then the elbow. As the elbow continues to drive upward, the arm extends at the elbow (with the racquet head still lagging). With this extension the forearm is moving faster but at full arm extension, the forearm is has slowed down quite a but in the upward direction. However, the forearm is still pronating after full extension. When the max forearm rotation occurs is dependent upon the type of serve. We also have some ulnar deviation as the wrist uncocks during the sequence that you've shown. So, in one sense the forearm and hand have slowed down (in the upward direction), but they continue to rotate. .