The Physics of Tennis

As I was playing tennis today, I became mindful of the physics behind the game. When I hit a slice shot, (a shot that has backspin on it), I produce the backspin by hitting the ball from underneath. I was aware that the reason why I don't have to hit this shot as hard as a regular topspin shot is because I don't have to change the direction in which the ball is spinning, only the direction of the ball's trajectory. The direction of its spinning remains the same.