Originally Posted by corners
The flip you're talking about is not done actively by Federer; it happens as a result of him beginning the forward swing with the racquet face closed. Having the racquet face closed at the end of the backswing with an eastern grip is only possible with a pronated forearm. Pulling from that position results in the weight of the racquet rapidly supinating the forearm and extending the wrist as the arm and hand are dragged forward from the shoulder. These forearm and wrist motions are passive, not active, and the result is that the weight of the racquet effectively stretches the forearm pronators, setting up the SSC action that is responsible for so much of the "free" or effortless RHS speed he gets.
Tricky, and I hope he makes an appearance, suggested many tricks to try to do this yourself. The most recent one, and the best in my opinion, is to pronate the hand of the non-hitting arm during the backswing - turning the hand/forearm so that your pinky rotates up toward the sky. Doing this with the off-hard tends to result in the hitting hand following this motion. This is a good trick, as you don't consciously try to do anything special with your hitting hand. We all know that consciously doing stuff during the stroke can get ugly.
I wouldn't obsess about my non hitting arm but rather keep the backswing on the right side of my body and keep my hitting arm relaxed and fluid.