The point must be higher after ball bounce off the table. If you start doing too early, the probability of error is maximum. Just try to play such ball on a training, not waiting for the highest point of rebound, and youll see it on your own.
If you start to do it in a moment when the ball begins to drop, then the error probability also increases significantly. With each lost centimeter a chance to perform a powerful top-spin drops because the ball is not high enough to fulfill top-spin to win points.
Remember: If you missed the moment of the highest point, then forget about top-spin to win. Make a simple top-spin, as if played with chopped or outflying ball. Yes, perhaps you would not win a point immediately, but you would take the initiative into your own hands, is this bad?
As you may recall the explanation concerning playing half-long balls with a marching on, we dont need to change the feet position here, since nothing prevents left foot to stand on the side line.
When executing top-spin, it is recommended to make a slight movement: left foot semi-step forward. This allows you to have more control over the ball and greatly increases the power of your top-spin.