This is the word I think was specially invented for tennis coaches to make them feel superior - "Do you know what pronation is"? you may ask a very important client of yours in the world of finance, and of course he does not know, making you feel so much smarter.
But watch Jim's video above and see if you really understand what pronation is, and what it invloves.
Many coaches tell their pupils tp "snap the wrist" which is of course totally wrong - pronation according to "word net search" is
(rotation of the hands and forearms so that the palms face downward)
Well of course we all know that, but in terms of serving a tennis ball, its all that plus the forward/upward movement which you will see on the video.
Also I have added this 3 photo series of Fernado Verdasco (I know he is a lefty, but I have turned him into a right hander!!)
Photo 1 shows the racket approaching the ball, edge ways on, photo 2 shows the first part of the pronation, as the racket face is turning to hit the ball, and the third shows how the racket takes its "up and outwards movement" which of course puts the rotation on the ball.Note also how the racket face is back to how it was in photo 1 - edgeways on!
Other points - free hand has moved out of the way, and is now tucked into his body - head up on contact - and look at the contact point, this will slightly alter depending on where he wants this serve to go - and the palm of the hand has taken an inside to outside rotation, not downwards!!
Watch and go out and try it - you know it makes sense!!