The point of these illustrations is to show that it is necessary to exert extreme force to keep your body stable in high speed rallies. Why? The centripetal force that is being applied to your shoulder, elbow and wrist joints during fast rotations is sufficient to cause small variations in the path of the racquet which in turn, are sufficient to cause an unforced error. As can be seen below, the players off set these forces by flexing every muscle to make their bodies operate as a solid unit. This may seem counterintuitive since one supposes that one must be fluid to strike the ball. However, it is only possible to do this when swinging slowly. The faster the swing, the more rigidly the player must hold their body, preventing the wrist, elbow and shoulder from moving the wrong way. The core and neck must also be maintained as ridged as possible. When the human body is held together tightly, the result is that it acts more like hard rubber and when it is held loosely it acts more like spaghetti.