Grip and Stance
I had a student last night who played his forehand in a nearly backhand position, facing the table. He also held the racket with his index finger almost down the middle, resulting in a floppy, wristy forehand. When I tried to get him to change his grip, his forehand looked like a jellyfish trying to do the wave. When I tried getting him to rotate his shoulders back on the forehand so he could use the whole forehand hitting zone, let's just say that too didn't work. It wasn't until I told him to change the grip and rotate his shoulders back that a little light bulb went off over his head, and suddenly his forehand came alive. We did about 30 minutes of multiball on his forehand, and now he's swinging like a pro. Well, at least in practice. It'll take time to incorporate it into a game.
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Fred Danner's Ping-Pong Diplomacy book