Here are some real insiteful throughts on why increased grip tension can prevent your improvement.
'The most overlooked and under-considered factor for players trying to achieve their potential in tennis is relaxing the hands. Not only can players increase their control by relaxing their hands, but they can also enhance their ability to hit with power'
'Having more control with a relaxed hand may be obvious to many, but it may be suprising to some that there is also more power to be had as well. It may be similary suprising that boxers often discuss the feeling of a loss of power when they tense up in order to try to hit harder. This reinforces the martial arts 'saying that' "power is in direct proporation to ones ability to relax" The relationship between tension and power is primaly due tot he fact that excess tension causes some muscles to work against each other'
Regardless of the stroke, players should relax their hands as completely as possible with only enough tension to avoid dropping their racquets. Ideally a player should hold on to the racquet very lightly , as if holding onto a bird, not so loose as to let it go, but no tighter than is necessary to keep it from flying away.