Vertical piano regulation - let off

When using this technique, use a gentle depression of the key, and never to the bottom of the key. If you depress the key to the point of the key touching the front rail punchings, the catcher could touch the back check. That will cause the hammer shank to move. Don’t let that movement get confused with let off. Also, there must have sufficient key travel to move the jack to the point of let off.