Johnstaf as the up-weight reduces when the hammer is in the backcheck
Yes, this is true from mechanical pov on full acoustic piano. But are we talking about hybrid or full acoustic setup without the strings?
I see hybrid now can have two implementation both of which require sensor on digital data flow.
I think you are talking about the full acoustic setup (with backchecks), which is only different from a real acoustic in that there are no strings (and the cast frame as they go hand in hand). But despite the backchecks there are no strings, the sensors are not there. I think the are under the keys but can have some variations, some might have hammer accel. sensors. Yes, the backcheck makes it slightly lighter to repeat, but this can be arranged with overall weight of the key – this is what I think is not needed. All that lever setup.
Indeed, that way I agree (partially). I guess I was referring to the 'cheaper' hybrid piano action where you have almost immediate sensor trigger (angled as shown) at the hammer's shank (they are hammers) here. Repetition‑wise, this setup is more consistent epecially if triple sensors are placed.
Otherwise, on an acoustic (or full hybrid) piano the re‑bounce of the hammer on forte+ dynamics causes it to get more gripped by the backcheck, that is how it is the opposite with piano- or mezzo dynamics. Although, I doubt that on full setup hybrids they can prove that the sensors are consistent with the hammers positions (free or backchecked). Just too many physical variables at play (pun intended)
Johnstaf Non-hybrids try to push the finger back up.
That is how it should be. Piano is a string percussive instrument. When you trigger a string it has its own mass and that is how the key of the piano should be thought of as a extremely big and bulky "plectrum". On a typical double escapement action (and full hybrid) there is just to many points of loss of energy + inertia from counterweights to balance the whole setup. The backcheck helps the repetition to happen faster and the jack to not get blocked by the knuckle (hence the whole point of repetition lever and spring attached to the bottom of the jack and the other side of the lever).
It is not about piano dynamics. Piano dynamics got lost long time ago, especially for faster tempos.
