Myth - Keep your wrist firm on all your shots.
Truth - The only shots I know of that requires a firm wrist are the volleys up at the net. On overheads and serves the pros snap their wrist forward and on groundstrokes they roll it upward with their forearm to produce topspin on the ball. Again we have one of those traditionally accepted axioms that just does not hold up under playing application.
'Keep your wrist firm' has caused many a good tennis player to become too rigid and mechanical on their groundstrokes. In stead of 'keep your wrist firm,' the instruction should be, 'do not snap your wrist to hit the ball.' Although in some situations you may have to snap the wrist in order to compensate for a ball that is behind you or when you are barely reaching it, for the most part you should not snap your wrist. Snapping the wrist will produce a flatter shot and not the desired topspin trajectory that a topspin produces to keep the ball in the court.
Using the wrist in an upward roll will provide topspin that will improve your groundstrokes.
Again, you must get away from conventional methods that try to stuff you into this rigid mold that produces robotic mechanical type players. To play with a more automatic, instinctive, and relaxed mindset you must abandon these methods. If not, you clutter your thinking with bits and bobs. I must do this, or I must do that on my shots to be a good player. You will try to squeeze yourself into this unrealistic mold that you think is necessary to play like a pro. The only problem is the pros themselves are not in that unrealistic conventional mold. Why in the world are you trying?
Maybe because no one has told you. Read some more of the myths and other information on this web site and you will become informed. Sign up for my newsletter and become brilliantly informed. Buy my books and tapes and you will know what most pros know!