ORIGINAL: turnnburn
During normal slow movements of the stick as would be normal during flight they have little or no bounce and jitter around neutral. But if allow the stick to center rapidly i.e let go of the stick they both have quite a bit of bounce and overshoot of neutral. It dampens out in maybe one to two tenths of a second.
When you let go of the stick rapidly, the stick does not go to center and stop, it overshoots the center and will bounce back and forth. The servo is just following the stick movement.