17 milliseconds before impact. Federer has stretched his ISR muscles using the various phases of the kinetic chain. Here he lets these muscles rapidly shorten, rotating his entire arm to accelerate the racket to just before impact. Probably about 70° arm rotation in 4 frames at 240 fps, the internal shoulder rotation rate is about 2800°/sec. Muscle shortening using pre-stretch is 'passive' and probably does not give the feeling of much effort. Impact was on the next frame, not shown.
The rapid wrist flexion may be a result of the ISR or earlier elbow extension and not wrist flexor muscles. ?
The racket 'edge-on' to the ball to the racket strings on the ball results mostly from the ISR turning the racket face as shown in these frames.
On the last frame just before impact, the angle between the forearm and racket is about 40° as viewed. (Probably a kick or slice serve.)
The angle between his shoulders and the upper arm is 145° (arm 35°
up from shoulder line extended) as viewed in this frame. That is a little more than most, 10-20°(?) up. See Ellenbecker video on the shoulder, serve and orientation to avoid impingement.
This is what determines the pace and quality of the serve. How can you see it without high speed video?