A Typical Upright Piano Action
When the key is pressed, it rocks on the centre rail, moving upwards at the back. In so doing, it raises the sticker and whippen. The whippen operates the jack, which in turn pushes the hammer butt. The butt pivots on its flange and moves the hammer towards the string. When the key is half way down, the damper spoon engages with the damper lever, lifting the damper off the strings. When the hammer is almost to the strings, the jack heel meets the set-off button, and as the whippen keeps moving up, the jack pivots and moves out from under the butt. The hammer then continues under its own inertia to the string, instantly rebounding. The balance hammer is caught by the back check and is held in this position as long as the key remains depressed.
When the key is released, the whippen drops, the back check releases the balance hammer, the bridle tape exerts a tug on the butt, and with the help of the butt spring, the hammer returns to the hammer rail (not shown). The damper spring returns the damper to the strings, and the jack spring returns the jack under the butt, ready for the next repetition. This entire sequence is momentary, allowing rapid repetition of the notes.