This Serve Will CHANGE Your Game | Topspin Serve Tutorial 

The contact time between the ball and the paddle is very short, about four thousandths of a second. In such a short contact time, the rotation effect caused by friction is not large. And when the ball leaves the paddle, waving your arms has no effect, because the paddle and the ball are no longer in contact, so what is the use of swinging the paddle following? In fact, the key is the rotation caused by the arc-shaped contact caused by the trampoline dent effect caused by the high-speed swing between the paddle and the ball.

The demo strokes while he's talking are illegal serve positions at contact with the ball.

not many comments here

I swear that second one you hit was out by 2 feet

The paddle is supposed to be below the wrist at contact. This serve is illegal.