The Jack and Let-Off Button are key parts of the escapement action. The jack (1) is held in its proper at-rest position by the jack regulating button (2). The regulating button has a felt pad (3) on the end that rests against the spoon. The jack must be positioned precisely in order for the action to function properly, the position of the jack can be adjusted with the jack regulating screw (4).When the jack is set in motion by the key, it pushes up on the hammer knuckle, sending the hammer towards the string. When the jack toe (5) comes into contact with the let-off button (6), the felt (7) on the bottom of the button stops the jack's upward movement, and rotates the jack on a flange set on the end of the wippen. The let-off button is connected by the button wire (8) to the let-off rail (9).
When the hammer rebounds and is caught by the back check, it compresses the repetition spring. When the key is released by the pianist, the spring pushes the hammer back up, allowing the jack to slide back beneath the hammer knuckle. The jack is now ready to repeat the process again.
