Originally Posted by Magic of tennis
YOu mean pros have 90 degree? 180 degree means you have to face back fence to create 180 degree if I am correct.
Nope, I'll speak for him and say he meant 180 degrees. Basically this means that after the unit turn, the outside of the back shoulder goes from pointing at the back fence to pointing at the net. That's a full 180 degrees of shoulder rotation.
Pros don't necessarily have that much rotation on every forehand, but sometimes they actually rotate that far or farther. Think of it as going from looking over your left shoulder (for a righty) to looking over your right shoulder on the finish. That ability to engage the torso and shoulder turn in such a manner gives a lot of power because the whole body is being used in the shot.