Topspin in itself, independently of the ball's forward speed, makes the ball bounce forward faster than it would otherwise. Picture a tennis ball flying from left to right. When the ball hits the court surface, the court exerts a friction force on the ball from right to left. If that ball has topspin, the bottom of the ball is spinning from right to left, in the same direction as the court's friction force; therefore the court's friction force presents less opposition to the motion of the bottom of the ball, and the ball slows down less. The opposite is true for backspin, where the bottom of the ball is spinning in the opposite direction of the ball's flight and thus of the court's friction force. Backspin increases the opposition between the motion of the bottom of the ball and the court's friction force; therefore the ball slows down more. If you've ever seen a drop shot bounce backwards, you've seen the ultimate demonstration of backspin's opposition to the ball's forward motion.
Topspin makes the ball bounce forward faster, thus diminishing somewhat the effect of greater height and steeper descent in increasing the steepness of its bounce. Backspin makes the ball bounce forward slower, thus increasing somewhat the steepness of its bounce. A drop shot will again serve as a perfect demonstration for backspin: Picture a drop shot that bounces twice in the same spot; it bounces as steeply as could be: straight up. In contrast, try a little experiment with topspin: Hold a basketball between one finger on each hand, give it some topspin, and then drop it straight down. A ball that is dropped straight down should normally bounce straight up, but the topspun basketball will instead jump forward.
For any given shot, the height, forward speed, and thus steepness of a ball's bounce depend on a complicated interaction of factors including many we haven't considered, such as rate of spin, fuzziness and hardness of the ball, and court surface firmness and roughness. The general results of topspin are quite predictable, though. If you hit significant topspin, you can hit with more forward speed at a given height. Your shot's faster forward air speed and faster forward bounce will reduce your opponent's time to prepare a response, its higher bounce will usually makes that response less comfortable and less powerful, and it will tend to drop into the court just when your opponent counts on it flying long.
