Topspin is generated in three ways:
- Swing from low to high, the upwards movement at contact causes the back of the ball to move up, causing topspin.
- Close the racquet face, as a ball hitting an inclined surface will naturally spin off of it.
- Rotate your arm through the contact (windscreen wiper follow through and pronation). These advanced techniques result in faster upward movement of the racquet at contact.
The big part of the equation that everyone seems to forget, is that if you are going to start hitting with more topspin you will need to change your shot trajectory. Here are two heavy topspin shots which are fun to try and good for getting a feel for how topspin effects the shot:
The Nadal style deep defensive/neutral forehand: standing quite far back, hit the ball with a fairly open face (still closed, but only slightly) aiming for 2m or more of net clearance and to then swing almost vertically up the back of the ball as fast as you can. If the ball drops near to the service line, you either had too little pace or you didn't aim high enough. If hit properly, this shot should be moving fast and high at the baseline, which makes a very hard shot for your opponent to attack.
Pay attention to the net clearance when he isn't going for a winner: http://www.youtube.com/watch?feature...JjrlnqFs#t=94s
Federer style aggressive forehand: Standing close to the baseline, aim just under 1m above the net and drive through with a more closed racquet face. Really let rip with a WW follow through, whipping across the back/top of the ball. The shot should be hit with enough pace that the ball lands half way between the the service line and baseline in spite of the heavy topspin and low trajectory.
Pay attention to the pace and net clearance when Federer *is* going for a winner: http://www.youtube.com/watch?feature...gL-1PNA#t=459s
(Note: Both players use different net clearance and shot pace in various situations, however it is often easier to think of the shot trajectory by associating it with the different players)