Another question from one of my videos on the chrisbrownrpt YouTube channel - Settle Compressibles and Refine (Jan. 2, 2013)
Q. How does aftertouch correlate to let-off? Is it simply the distance the key travels to its bottoming out position once escapement has occurred?
A. Yes, aftertouch is that distance between letoff and full dip, but "simply" undersells the important content of that extra travel. For a start letoff spans the moment of jack tender contact with the letoff button through the jack tip in a sliding exchange with the knuckle up to the moment of disconnection from the knuckle. Technically, at that disconnection, the note will play, if it happens before hammer crown connects with piano wire. But further travel is needed for the note-playing finger to feel comfortable in its landing and confident that the next note will not jam or bobble.
Letoff disconnects the driving force from finger-key-whipcushion-jack (supported by balance rail-keybed) from the knuckle-shank-hammerhead so that the hammer arrives free to set strings in motion and bounce away. Without aftertouch, the player would need to back fingers away staccato fashion to not jam the mechanism into the strings, severely limiting the range of motion needed for nuanced, musical playing (which has a component of aim and throw - think what pitching a baseball or hitting a tennis or golf shot would be like without follow though).
So, aftertouch is the safety margin, the follow through, to assure that bounce with no taint of jam is what takes place. The key travels after letoff through dip to its stop, the front rail (supported by keybed) buffered by punchings. And the jack clears the knuckle, but not so much as to jam against the repetition lever.
And the backcheck continues to climb, with enough follow through to secure the hammer's tail exactly right as the hammer bounces away from strings now set in motion.
Meanwhhile, the drop screw gets the repetition lever stopped and out of the way for the bounce to be free of rep lever influence, and the repetition lever spring is compressed, flexed to speed the lever's return to a height that allows reset clearance for the jack as it is released by the jack tender and letoff button disconnecting.
In the background, the key's motion beyond letoff also helps clear the damper from the strings about to be set in motion.
The follow-through provided by aftertouch makes playing more comfortable for the player - the less aftertouch, the more fingers feel as if they're playing on a board (which really they are - the keybed), instead of into sound. And, if very even, aftertouch improves player control, accuracy, and the ability to make the piano sing!