To execute a forehand topspin, scrub the paddle upward against the ball, while the face of the paddle is tilted slightly forward. In the video, the motion is exaggerated.
This is a maneuver that should executed when you feel you have a strong advantage in a kitchen rally. If you try this too soon, without enough advantage, it will most likely come back to you even faster, beyond a speed at which you can react.
You may have gently bounced the ball back and forth a number of times with your opponents. You patiently waited until they were out of position, or gave you the ball a little too high. That’s when you use your forehand topspin. One of the best places to put this is directly between the two opponents.
This has several advantages over a flat shot. First, it is fast, and goes low to the players’ feet. It will also clear the net when a flat shot won’t, because the ball is slightly lifted and spun as it is hit, giving it an arc over the net. Finally, if it hits the tape at the top of the net, it will have forward spin traction and is more likely to roll across to the opponents’ side, than to fall on your side. And, once it hits the tape, it will pop strangely, being nearly impossible to hit back with any accuracy.