It is really important that you can control what spin you put on the ball whether it be slice or topspin. I hear you ask 'Who cares which way the ball is spinning as long I get the damn thing in'. Controlling the spin will allow you to dictate the point better and give you more purpose behind the shot. So why topspin? Well topspin will allow you to hit the ball harder and still get it in. Not wishing this to be a physics lesson (I was useless at physics - just thought I'd throw that in) the spin will make the ball arc over the net giving you more clearance over the net and make it dip quickly the other side. Also it will make the ball kick the other side making it more difficult for your opponent. So how do you produce it? It is important to drop the racket below the intended contact point. Then, and this is the important bit, with the racket face facing the net you need to brush up the back of the ball and finish you swing up over your shoulder. Contact point should be no higher than waist high. Now have a look at the rotation of the ball as it goes over the net. Is it going end over end. Yes? Perfect! No? Try again. Once you master it, it will change your game.