https://youtu.be/0OBctGfK09s?t=56 .....(may have to copy and paste for now)
I do like his presentation and use of the topspin tool. He does an nice job, but it is a low resolution perspective of how best slices and serves in general are hit.
The difference is subtle and slight, but very Key.....For good serves with more serious power instead of just brushing it over the net into the box, you hit more thru the ball instead of brushing across it. Notice I said "more thru the ball" and not directly thru the ball. The racket path should go "thru" the outer half of the ball to a greater or lesser extent and travel thru that edge at about 25-45 degrees off of the intended ball path....depending on your spin/power mix intentions. Driving thru this section of the ball is what P.A. Vaile meant when discussing about "hitting a point on the ball". This type contact will not only create the torque on the ball you seek to create spin, but it will give you access to many aspects of creating power in the process. His "hit, then turn" doesn't do this as effectively. For him the pronation is merely a result...not an amplifier that it is for top servers.
One of these advantages is that your racket can stay on edge longer, while still getting the action that 'most call pronation' during the contact. This twisting of the racket during and thru the contact is sort of an extra jolt of power to the serve at just the right moment where it is least resistant to this acceleration. Another aspect of power here is to hit thru half the ball is a bit like increasing the weight of your racket, since it is only attacking a portion of the ball, thus increasing the racket's mass advantage over that part of the ball. This allows the ball to blast thru the contact with less loss of acceleration at contact. It's a bit like how you can clip another car on the corner and drive right on thru (even spinning the car), but if you square up on the car it will like grind you to a halt. If you sideswipe down the side of the car like this video, then you will likely achieve no spin of the other car. I realize the ball will spin with this brushing sideswipe, but the car example illustrates how much stronger ripping thru an edge is by comparison.