Originally Posted by Raul_SJ
Keeping the hitting arm palm down avoids the "waiter's tray" position and gets you to the racket drop position smoothly so that you can then accelerate up into the serve for power.
The palm of the hand stays down on the take back and then comes up like you are going to salute.
Check this video.
Yes, I have always thought the "palm down" was to basically assist with keeping a closed racket and avoiding opening up the stringbed in the waiters tray. My view is it should help with spin and maybe a little power. Spin as it should result in you approaching the ball on edge allowing a good brushing action. Power if you open the face to hit flat as you have more distance to pronate from closed to flat at contact but mostly spin.