Necro-reply -

When you chatter an opposite flipper you cause the rolling ball on the stationary flipper to rattle, vibrate and sometimes hop a little. Its usually very subtle and sometimes not so subtle.

The rattling ball loses some of its normal force pressing into the flipper rubber and reduces the efficacy of any spin that was helping the ball roll up toward the tip of the flipper. (sometimes the ball even pops off the flipper rubber reducing the effect almost entirely).

...imagine spinning a pinball on a flat table top like you might spin a coin. The spinning pinball can stay in one place but is spinning rapidly. Now the moment you touch the pinball it catches on your finger and goes shooting off the table. That is whats going on in a pinball machine, the table is the playfield and your finger is the flipper. You want to reduce the amount they interact.

-cAyle