In my opinion though, on most piano the tone isn't as nice with the una corda...on my teachers Steinway it has a very odd tone. Doesn't it also mean that the hammer only strikes 2 of the 3 strings?
I think it hits only one string if you push it all the way down, two if you push it halfway (think of Beethoven's 'poco a poco tre corde' direction). For sure this will change the sound drastically, losing the subtle interference of the 3 strings which normally determines the character of the tone. It is really a strange design, come to think of it. Somebody should build a grand with two sets of hammers instead
In response to pianolady's question (which I only just noticed) : No I never use the una corda pedal, although I find it difficult to create a true pianissimo, and perhaps should start using it in very selected places. But as MaxW says, the tone sort of falls flat. So hmmmm, I'm not sure.
No, the una corda pedal on nowadays grands work so that one hit 2 strings if the pedal is pushed all the way down (For the 3 strings notes). For the 2 strings notes, one hits 1 string with una corda pedal. For one string notes, nothing changes regarding number of strings, whether the una corda pedal is hit or not.
I think the una corda for grands (not to mixup with soft pedal on uprights - complete different behaviour (the hammers will only be put closer to the strings)) is really an ingenious invention. Since the string numbers which come in contact with the hammer is less, the sound gets softer - however not very much, since the empty string will vibrate too, even without beeing hitten. But the second effect is indeed, that the hammer comes with a different felt part in contact with the strings, what is not so worn out. That's why it sounds not only a bit softer, but foremost mellower. This can be a very drastic difference.
I too find it difficult to create a true pianissimo (but worthful to try more and more), but that has not so much to do with the una corda pedal. One can play forte with una corda pedal and pianissimo without una corda pedal. But the sound change is drastically, and one robs a lot of sound change possibility if one don't use the una corda pedal. I use it for Chopin playing often, for Bach playing only occassional. It depends also on the grand I play - if it is a grand with worn out hammers, what sounds harsh, I would use it much more than on a new intonated grand.
There is a trick, if the hammers are terrible hard: One can put a piece of wood (ca. 1 mm thick) at the end of the action, so that if the una corda pedal comes back, it stops a millimeter sooner. So one hits also during normal playing all strings, but on different hammer felt place. It will sound much more mellow now, not harsh anymore. Caution however: if the hammers have deep groovings, there is hammer torsion created this way, because on hits the strings on the side of the grooves, so better consult the piano technician for that!