In respect to this, what is a backhand topspin?
The backhand topspin is an attacking shot used to put your opponent under pressure, it is a progression of the backhand drive where we look to generate more spin and speed on the ball.
Herein, why would you use a backhand topspin in table tennis?
A backhand topspin is more consistent attacking shot compared to a backhand drive, because the spin lifts the ball over the net and brings it back down the other side. This means you can attack balls which are very low, something which is difficult to do with a drive shot.