Originally Posted by watungga
My diagnosis, Fed's pronation from this point out is just all follow-through. The follow-through creates the pronation, not Fed exerting pronation. His racquet's swingweight takes the job of providing impact/energy to the ball.
If you look at it with a super high speed video, you'll notice that he begins to pronate before impact. Furthermore, comparing with the more common pro of the tour, you'll realize Federer pronates a lot more quickly.
The phenomenon which breaches the gap is called "stretch-shortening cycle" (SSC). Basically, by putting his hand and forearm in a specific way as he swings forward, his forearm pronators get extended. Following this extension, he has a 0,5 seconds window to benefit of the SSC: if he contracts in that window of time, his contraction will be much faster than usual.
For most movements, you might do it naturally. Most people first take the ball back before throwing -- that's an habit which was ingrained by the efficiency of the different results: pulling it back and then quickly forward enable the kid you were to throw harder and, as such, you accommodated your tossing action to throw harder. It's also this sort of reflex that we use when using the split step: bouncing down extends our quads which allow us to move faster off the first step.