I saw this on a 2-pedal upright piano. The owner showed me inside how a strip of material (felt) had been added so that the soft pedal had a much more pronounced effect, and explained his piano tuner had done this for him because lacking a practice pedal, it was too noisy for his neighbours.
Is this a common trick and is it considered OK or some sort of piano-blasphemy?!
It's how the practice pedal, when fitted originally, would work. Sometimes, the pedal moves all the hammers sideways, in order that one string out of two or three are not hit, and the lower hammers sort of glance off the side of the strings (from memory). So, it's not a problem, and could be removed easily when the neighbours have gone!
The high strings may have the hammers brought closer using the 'soft' pedal, which means they could be tripped more easily by the player pressing keys.
The "practice" pedal is often fitted on upright pianos as a third pedal, between the usual two. It brings a strip of felt down between hammers and strings, and has the obvious effect. Where a third pedal isn't fitted, there may be a manual method of swinging the felt into position. I've never seen it usurping the soft pedal function, but I suppose it's feasible. In studio pianos (that's recording studios) the same mechanism sometimes brings metal strips down instead, for a "jangle" bar-room piano effect. Less drastic than the alternative - tacks pushed into each hammer.
