Sympathetic resonance is a subtle phenomenon that occurs in real pianos when the sustain pedal is pressed. Normally, when a piano key is not being played, a felt damper presses against that key's strings, keeping them from vibrating. However, when the sustain pedal is pressed down, those felt dampers are removed, and all of the strings of the piano are left to freely vibrate. When keys are pressed and notes are played, other nearby strings, because they are no longer dampened, start to vibrate in sympathy with the strings that have been played. This causes a slightly fuller piano sound to emerge, as more strings are vibrating (or resonating) around the ones that were actually played. When the sustain pedal is released, this Sympathetic Resonance sound is immediately muted.
Sympathetic resonance is a very subtle effect, and it's only audible under certain circumstances. Sympaethic resonance adds a subtle shimmer to the sound of notes when the sustain pedal is held down. If you press several keys, then press the sustain pedal, you may hear the subtle shimmer of sympathetic resonance fade into the sound. When you release the pedal, it will fade away.
If you cannot hear this effect, try turning the sympathetic resonance control all the way up. You can find this adjustment on the Tone Controls page. Once the control is all the way up, press several keys on the keyboard, then press the sustain pedal. You should be able to hear the sympathetic resonance effect fade in. Now, dial back the amount until it is barely audible. This is a good approximation of the level of this effect in a real grand piano.
If you don't like the way it sounds, set the sympathic resonance amount to zero.