• Because tennis is about using a racquet to apply force to a tennis ball.

As you swing the racquet about, your body is reacting. This "equal but opposite" reaction, however small,  can cause your entire motion to go off course without you being able to perceive it. The consequence is that, even if you move the racquet to the ball in a nice straight line without rotation, the shift of the body or other parts of the arm may cause you to hit off center. 

The remedy is to learn a few "tricks" to keep the equal but opposite reaction (or EBOR) from messing with your stroke. 

The easiest trick is to lean forward slightly in the direction that your racquet will move over its last four inches as you swing. The lean offsets the swinging force without throwing your racquet off its straight-line path. The reason is that the action of the arm in a forward direction can only produce a slight reaction of the whole body in an opposite direction because the body is much bigger than the arm.

However, if you are swinging while standing with your weight on the back foot and standing up straight, the forward thrust of the racquet toward the ball will be enough to tip you backward. Falling backward, even a little, can be enough to give your opponent an advantage over you. We will analyze this advantage/disadvantage in a later lesson. 

In the figure below, the subject started leaning forward before striking the ball. The harder you want to hit the ball, the greater you will need to lean to avoid tipping backward. From all appearances, the subject below is striking the ball at a high velocity.

High-Tech Note: Stability Your motion is stable if the EBOR does not cause your racquet to deviate from its intended course. A key to this is the placement of the elbow in front of the body and not too far to the side. You must experiment with the elbow position to find out what is stable. Most people cannot, at first, "feel" the difference between having the elbow at or behind the body versus in front of the body during the swing. So your initial experimentation may seem uninformative. But this will change in a short period of time if you persist in experimenting. Why? Because the brain has initially allocated very few sensory neurons to provide "feeling" for the elbow movement. If you experiment, it will allocate more, and thus your feeling will improve shortly.