Please slow these to min-speed and have a look at 1:43:
https://www.youtube.com/watch?v=BZ9nKEFSib8
0:41, 0:48, 0:51, 1:43 (best)
Other videos:
https://www.youtube.com/watch?v=IJ_8tEhY_Ck
https://www.youtube.com/watch?v=IiARkUO6aEE
When he hits the ground with the right foot, the movement starts. Technically both hands start moving, but the free arm is there at start moving more, for you it feels as if the playing hand is still frozen, and the free-hand is already moving, like preceding...
So you hit the ground, it goes to the leg, then hips start rotating, then upper body starts rotating, and with it the free-arm. It all happens at once. And for a short while it's like the playing-arm is not moving (well it is, but it feels as if not moving). And then when the upper-body is already rotated, the playing-arm shoots, like with a delay. And since it needs to make up for this delay, it shoots faster. I don't have more precise words.
Btw. even though some people criticize TB for chicken-wing, this mechanics is also what he does perfectly, imo.
Edit: I think this preceding happens also in usual top-spin, and gets more and more prominent as you add the power, like loop-kill on backspin.
Lovely. I sorta got it. I was more about how that extra shoulder pull works. That 1.43 is quite amazing yeah.
Regarding the arm “delay”, it’s secondary acceleration that we discussed in another thread with Zeio’s Chinese measurements of top players. In term of execution, all you gotta feel is that you rotate your body into the ball, then at the moment of contact (or to be precise, just before it), you snap the arm. It has to work like that yes.