@zill
On an incoming ball with underspin (US), a volley can sometimes be a tricky proposition. In this case you need to reverse the spin on the ball in order to put your own underspin on the ball. If the incoming ball has TS, then your US will be in the same direction that the ball is already spinning.
But the situation is different on balls that have bounced. Pretty much
every ball that hits the court, whether it had TS or US prior to the bounce,
has TS after the bounce. Therefore, it requires less effort, less energy, for you to impart US on an incoming ball that has bounced.
Some people believe that a ball that bounces backwards, had US after the bounce. However, upon close inspection, it becomes apparent that this is not the case. The ball still has TS -- but in the reverse direction. TS & US designations are relative to the direction the ball is moving.
The ball can bounce forward, to the left, to the right, or even backward. But in pretty much every case the ball has TS in its new direction. There are a few
cases where the ball has almost no spin at all but, for the most part, it will have TS after the bounce.