Turn the Doorknob, Twist Open the Jar
Doug King's article, "The Hands Have It", was a revelation when I read it several years ago, but it wasn't until I recently started capturing footage at 1000 fps, in high definition, that I could fully appreciate what Doug was describing in that article. The above quote is just a small piece from the larger article, but Doug's analogy of using the hand to turn a doorknob to generating topspin on a forehand is what I want to bring to life in this article, and perhaps build on.
If you turn a doorknob backwards and forwards you'll get a great sense of how the hand works coming into and through contact. I like the idea of the doorknob more for the backwards twist because there is no resistance when you twist back. Only the weight of the racket in the hand. However on contact, there is tremendous resistance. The ball is aggresively pushing into the racket and you must turn the ball with more of a muscular force in order to counteract this pressure and apply a forward spin. That's why I like the analogy of opening the lid of a tightly closed jar. It's the same counter clockwise motion as the door handle, but you need to engage the forearm muscles in order to turn the ball. I also like the analogy of turning a wrench in a counter clockwise motion. Both motions generate torque by applying force from the turning wrist and forearm muscles.