Grand Action 
Rest position The hammer rests on the roller (knuckle) on the repetition lever, which sits on the capstan & key. This hammer/repetition assembly holds the key in its ready position. The weighted damper rests on the string. 
Let Off The jack's position is changed by the escapement dolly, & the top of the jack is kicked out from the hammer roller. The hammer, now under its own momentum, continues towards the string. The repetition lever is stopped in waiting position by the drop screw.
|  Acceleration
When the key is depressed, the repetition rotates upwards. The jack pushes the roller and accelerates the hammer. The damper is lifted off the string by the far end of the key.
Checking The rebounding hammer falls back onto the repetition lever, in front of the tripped jack, & is caught at the tail by the check. The stroke may now be repeated, either by releasing the key as usual, or by using the double-repetition feature. |