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I am going to demonstrate the reasons why is is important to bend your knees when lunging for the ball. Often the ball is quite low, near the floor, so if you bend your knees as you lunge the racket head will drop together with the wrist. If you bend from the waist then the racket head will drop below the wrist and cause you problems when hitting your shot.
So in this position, ready to play my shot. I'm not going to move my racket arm. I am just going to bend at the knees and as you can see my racket is still aligned with my wrist. Soon as I bend from the waist you can nor see the racket head drops below the height of the wrist. I am also over the ball and also close to the ball which makes it all the more difficult to play that quality shot.