The wrist was built to make highly variable adjustments when handling objects like a fork or for power to throw a rock. When a tennis ball approaches a first time player, their natural response is to use their wrist to hit the ball at whatever height it arrives. The problem is that the player then has an infinite number of contact points to choose from when using the wrist and none can be optimized for accuracy and consistency. It would be like going to the driving range to learn how to hit a golf ball except that you would tee the ball at a different position and height every swing. If that were the case you would never get better at golf. The same principle applies to tennis.

With the wrist blocked, your practice time becomes dedicated to positioning and finding the optimal contact point every time you hit the ball. Once that technique and response is ingrained into mental memory you will never think about how to hit the ball again and naturally position yourself better.

The goal is to approach the ball the same way each time and hit at a consistent contact point. This is inherently difficult as long as the brain can use the wrist to make the adjustments for you. By blocking the wrist, the brain automatically focuses on positioning the body first which trains the brain to use the feet to make adjustments and for power.