The touchweight adjustment enhances the potential of a piano touch in a way rarely achieved previously.
Generally speaking when touch problem appears, a thorough regulation is enough to solve this, but it happens that the problem remains because the issue is the part itself.
This occurs mainly in older pianos which had new parts installed (even though it happens to new pianos at a certain extend).
During the course of a piano’s life, some parts become worn and are replaced by new ones. If those parts do not have the same weight and shape as the original, the touch is affected.
It is possible to compare the action of a piano to a mechanical scale. A weight positioned on one of the plates, pushes the lever down. In the piano action the principle is the same. There is the key on one side and the hammer on the other extreme of the lever, with the pivoting point somewhere in between those extremities. If a part is replaced in that system and its weight is different from the original one, the touchweight changes. It become lighter if the new part is lighter, and becomes heavier if the new part is heavier.
Of course a piano action is somewhat more complex than a simple scale, but the fundamental principle is the same. If you press a key, the other extremity rises (the hammer). If you release the key, the hammer falls back to its resting position. It is a balancing system.
Someone once asked me why the damaged part is not replaced by the right one to start with.
Well, the answer is not that straightforward. There was not always a large choice in piano parts. The technician had to adapt with what was on hand. There is also the fact that piano factories did not deal with the private aftermarket. For that reasons, many piano rebuilding has been done empirically in the private line. That said, there are very talented private workshops too, of course. Fortunately today, we have part replacement choices and the possibility to improve our technical skills thanks to specific training possibilities.