Yeah so basically you can either rotate the entire arm when you arm is up, or you can rotate just the forearm, to try it out hold the racket hand straight up in a forehand grip and the racket should be parallel to the ground and the forearm supinated almost completely. From this position you can see that you can either rotate just the forearm or you can do the entire shoulder(pronation + shoulder), so which one is correct? During the entire shoulder rotation, you can see that the elbow changes the way it's facing(by a lot) whereas during just forearm pronation the elbow doesn't change the direction it's pointing in(much).
The minimum done is a "stick smash". The elbow is kept at about same height throughout the motion. Both upper arm rotation and forearm pronation are done. It is never only a forearm rotation. And besides the two mentioned rotations, you can add grip tightening too.
