I noticed coach was training my son to use his shoulder to get more power in learning the forehand drive. Watching some Youtube videos of Ma Long, it's evident that he uses his shoulder very much. First, he uses his left hand to point towards the ball, this helps him twist his left shoulder towards the ball, then he pulls his left hand retracting it to his elbow. This helps when he hits the ball, his right shoulder twists around to his left and finishes up almost pointing towards his opponent.
Of course, the shoulder twist is most prominent during strong or powerful shots, not when he is warming up or just blocking the ball. I hope coach can teach the kids to effectively use their shoulder in their shots. Coach also said that the hip, knees and feet rotate as necessary to give optimum power to the shot. This is not happening yet for my son, but Rome wasn't built in a day!