With groundstrokes, your spin options are topspin and slice.
With topspin, the ball dips to the ground quicker and bounces higher than you'd expect from a ball with no spin. It allows you to hit harder - and higher over the net - with less risk of hitting long. It also allows you to dip balls at the feet of an incoming volleyer. Use it as your stock rally ball, but flatten it out a bit to go for winners. Topspin is a player's insurance policy and you shouldn't be allowed to drive without it!
With slice (or backspin), the ball floats through the air longer and bounces lower than you'd expect from a ball with no spin. It allows you to hit deeper and lower, forcing your opponent to hit up on the ball - very desirable if your opponent has an extreme "closed" grip or you're approaching the net for a volley or an overhead. Use it as an approach shot and use it for defence when you've been stretched out of position, because the ball will travel slowly and buy you time to recover for your next shot.
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