Why Can a Silently Held Key Join the Resonance?

On an acoustic piano, slowly depress a high C all the way without making its hammer strike. With the other hand, play a low C firmly and release it quickly. You may hear a faint sound continuing from the high string. It was not secretly struck. The slow key movement lifted that string’s damper. The low C contains a related harmonic, and energy travelling through the soundboard and air can set the now-free high string into sympathetic vibration.

This matters not as a performance trick, but because it shows that piano sound is more than the notes directly struck by hammers. The moment a key is held, the moment a damper leaves a string, and the frequencies that are closely related all shape what happens after the fingers move. The sustain pedal lifts many dampers at once and extends this relationship across numerous strings. Holding a single key with a finger makes the boundary of the resonance easier to hear.

In practice, do not chase a spectacular effect. Compare two conditions—high key released and high key silently held—and wait until the low note has disappeared before judging the residue. Different pianos, rooms and attacks will change the result. Whether a digital instrument reproduces it depends on its sound engine and resonance modelling. The point is not to prove a mysterious connection, but to hear damping, harmonics and decay as part of musical structure.

https://www.yamaha.com/en/musical_instrument_guide/piano/mechanism/mechanism003.html