Hi All,  I'm not a physics anything but I do know this much:  the
hammers in an upright action pass over the center of balance point
before they hit the strings.  Anyone who has spaced hammers to the
strings before the hammer return spring rail is installed knows this
perfectly well.  This is due to the fact that the hammers are 'front
heavy' and is part of the reason for the existence of the hammer return
spring.  Naturally, once the hammer return springs installed and are
working properly, they apply enough return force to push the hammer
back across the balance point.

So with all that in mind, I have to agree with Ray Fairfield that there
is no possible way for the hammer to increase speed once the finger has
left the key _or_ after 'let-off' has occurred.

By the way, where did the term 'set-off' come from?  I've never seen it
in any of the reference books I have for piano actions.

Musically,

John A. Tuttle