Killing the Spin
Naturally enough, some rubbers will be better at killing the spin than others - this is why you very rarely see a normal rubber player wobbling the ball - in order to completely kill the spin he would have to exactly match the spin on the ball from his opponent - too little and the ball will still have some of his opponent's spin, too much and he will put his own spin on the ball. Only if he gets it exactly right will the ball have the chance to wobble.Long pimples are naturally better at killing the spin than normal rubber or antispin - this is because when used correctly, the long pimples will bend around the ball and tend to grip it from all directions, so when the ball comes off the pimples they will all be trying to spin the ball, and thus the overall effect will be a float ball. I've got a detailed explanation with pictures of how the long pimples can kill the spin here.
Maximizing Your Chances of Wobbling the Ball
In a nutshell, your best method of creating a wobbling ball will be to use a long pimples with pimples that can bend enough to grip all sides of the ball, and then attempt to hit the ball in a relatively flat stroke, or with a stroke going against the spin of the ball a little. This also explains why the stiff pimples with glassy, frictionless tops but grippy sides are better at creating wobbling balls from hard loops then from softer shots - they need a certain amount of force to be able to bend the pimples around all sides of the ball - without enough force the pimples won't bend and the ball will tend to keep spinning.So a softer, more flexible long pimples will tend to better at creating wobbling balls from all situations, while the stiffer pimples will tend to only work against shots with a certain amount of force. You could probably try to hit the ball harder yourself to cause the stiff pimples to bend against a soft shot.
As I said earlier, this is only my theory of how long pimples can create a wobbling ball, so I'd love to hear any other theories you might have - and I'd be more than happy to add them - so please feel free to email me with any ideas of your own.
(Note: Bob Sollish has provided his own detailed explanation of why a table tennis ball wobbles, involving fluid mechanics and more.)
