With underspin, the vacuum forms at the top of the ball, and 'sucks' the ball upwards. The same principle applies with sidespin, except the vacuum forms on the side of the ball, sucking it left or right, depending on the spin put on it.
Also, a slight vacuum forms at the rear of the ball, due to its motion. There is no technique that can overcome this, it's the nature of anything in motion (i.e. even a snail sliding across a leaf will have this vacuum). The only thing that can be done is to use a new ball.
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Next: Return to the Basic Physics and Mathematics of Table Tennis/Ping-Pong - The Physics of Reaction Speed
