To generate topspin, your racket AND ARM must move straight up the back of the ball as you push forward. The entire arm, from hand to shoulder, must lift upward as the wrist stays stable. When you do this correctly, your racket will follow the path of the above animation, moving forward and upward (rather than across your body). It is critical that you keep your wrist back as you lift your entire arm upward from the shoulder. In all the animations on this page, you can see how the pros lift their arms and rackets upward together. By working the ball with not just your racket, but with your entire arm - from shoulder to hand - you will generate the "heavy" ball that top players produce. It's simply a matter of maximizing support and mass behind the racket as you drive up the back of the ball.

Because the racket actually holds back as the arm lifts upwards, we see that the role of the racket is to hold and grip the ball while the arm provides a solid counterforce that pushes and lifts upward. This discovery, that the role of the racket is not to hit the ball but to grip it, was discovered by Doug King. I find it to be the most profound insight into the modern game. Let me quote Doug:

"In the modern game, the swing of the racquet is minimized. The result is better contact between the racquet and the ball - that translates into less slapping into and pulling off the ball and fewer off center hits. The hand positions the racquet to the ball and allows a leveraged drive and turn of the ball initiated with the body."