I have 2 racquets, one is U and another one is 2U. I normally use the heavier one for warming up, do some strokes and smashes. I will shift to the lighter racquet when playing in a game. I do this because I lack of strength and power all the time and I find by doing this way, my power enhanced after I switch to the lighter racquet. Does this just psychologically influenced me or it does have some physical effect?