Tighten up the muscles in your wrist and see how fast you can snap your racket. Then release the muscles in your wrist and see how fast it will snap. Clearly, a loose wrist is more flexible. When serving, power is generated, at least in part, by the flexible snap of the wrist. If you try to hit hard intentionally, you are likely to over-tighten the wrist muscles, slow down the snap of your wrist and thereby lose power.
