Look through almost any table tennis instructional book or video, go to almost any table tennis camp, and you'll hardly ever see or learn anything about topspin defense. (Also known as "fishing.") It seems to fall through the cracks for most coaches - it's not quite lobbing, but it's not your basic forehand or backhand drive or loop either. As a result, it is often shunted aside and ignored. But watch any tape of the best players in the world and you'll see topspin defense over and over.
What exactly is topspin defense? It's a halfway shot, half lob, half loop or drive. It is done from off the table, against either a smash or a loop kill. The ball crosses the net perhaps one to three feet high, sometimes higher or lower. It should have at least some topspin, and sometimes sidespin as well. The ball should land deep on the table, and bounce outward due to the topspin. It should be done against an aggressive drive, loop, or smash.
Topspin defense is easier and more effective with inverted rubber, but it can be done with other surfaces as well, but with less spin.
Although lobbing is a type of topspin defense, topspin defense is generally more effective if the ball is kept lower. This way the opponent has less clearance for smashing, with the ball bouncing mostly outwards instead of up. How does one execute topspin defense?
Start off from as far from the table as necessary to react to the opponent's attack. As the ball approaches, start with racket just below the ball, and use a mostly forward, slightly upward stroke. Contact the ball on the back and just "fish" it back. If you have a good loop, just think of it as a soft loop. If you are a good counterdriver, think of it as a counterdrive with a little extra topspin. Try to keep the ball deep, relatively low, and with some topspin.
Depth is key. If your shot lands short on the table, the opponent will be able to cream the ball at wide angles, often right off the bounce. But if you keep the ball deep, it's low percentage for him to do that, and since you have more time to react against a weaker ball that's not wide angled, you can get a lot of balls back. Make sure to move the opponent around to increase the chances of a mistake - either missing or a weaker attack.
However, the goal of topspin defense isn't just to wait for the opponent to miss - it's also to find a ball that you can counter-attack, usually from your stronger side, which for most is the forehand. (The body is less in the way on that side, so it's easier to counter-attack.) It's sometimes good to fish into the wide forehand, so the opponent might attack into your forehand.
To fully appreciate topspin defense, you have to see it in action. Watch a video of some of the best players. The rallies are spectacular!