3. When a key is pressed on a real acoustic piano, the hammer is flung at the string with a velocity proportional to how hard the key is pressed. If the key is pressed very lightly, the hammer velocity will be so low that it will not reach the string before falling back to rest, and no sound will be made. If a note is played normally, and if the pedal is held down so that the damper for that note is not allowed to fall, then replaying the key very lightly should not interrupt or disturb the continued ringing or decay of the note. Basically, this test swaps the two different ways one can lift the damper on a string that is playing, and does so in an overlapping manner so that the string should neither damp nor replay. Digital pianos often duplicate this behavior - I call this "silent replay".