Myth - The racket turning in your hand means you did not hold on tight enough.
Truth - Let's see, they tell me I should be diplomatic so I do not offend anyone when I write these things. Let me give it a try. Here goes! Ninety percent of the time the reason why the racket turns in your hand is because you are judgmentally challenged. How did I do? What do you mean you are not even sure what I am talking about!
Okay, how about this. Ninety percent of the time the reason why the racket turns in your hand is because your judgment of the ball is poor and you did not hit the sweet spot of the racket. Not politically correct, but it is true! Do not feel bad though because the same is true for the pros.
When they miss the center of the strings the racket will turn in their hand also. The pressure that is created when hitting off center is tremendous and causes the racket to turn in the hand.
The true answer to stop the racket from turning in your hand is to improve your judgment of the ball by hours and hours of practice. Do not try to hold the grip tighter to stop it from turning. This may be one of the contributing factors to 'tennis elbow.' Everyone is holding on so tight it is placing strain on the whole arm!