What is a Piano's "Action"?
The mechanism of the piano that causes hammers to strike the strings when a key is pressed is called the "action."
When one speaks of the history of the piano action, mentioning the repetition mechanism (double escapement) invented by Sébastien Érard of France is a must. This mechanism allows the pianist to quickly repeat a note without having to fully release the key. Up until the introduction of this mechanism, when a key was depressed, the hammer usually rose and struck the string and was not ready for the next keystroke, until it had fallen back to its at-rest position. Erard's invention made it possible to prepare for the next keystroke even though the hammer had not completely fallen back to its at-rest position.
The damper mechanism is another important part of the action. This mechanism quiets the sound instantly as soon as the finger is lifted from the key. In the photograph, the four white parts are the damper felts. When the finger is lifted from the key, the dampers touch the strings from above and stop the strings from vibrating. A damper weight is attached to the bottom of the long vertical wire.