h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Wonderful! A most excellent entry. As a self-taught pianist I've only ever really used the sustain pedal, and am satisfied that my instinctive use and timing enhances the effect.

However, my Yamaha upright has the following pedals: right=sustain, left=una corda, middle=some sort of other damping pedal. I seem to be short a sostenuto! I've had the panels off the piano to watch the mechanisms in action, and the effect of the middle pedal (which also has a locking position, by sliding the pedal sideways when it is depressed) is to lower a strip of felt between the hammers and the strings, thus producing a dulled, or damped effect. I find it very handy to lock this pedal in position for practising while my children are asleep! The result of depressing the left pedal is as you describe - i.e. to move the hammers closer to the strings, thus decreasing the impact.

Any light to shed on Yamaha's mystery strip of felt?!

The "mysterious strip of felt" pedal is often called the "practice pedal". It is sometimes offered as a third pedal with the damper and the soft pedal. It is the only one of the four possible pedals that can be locked into position.

The sostenuto pedal is only found in grand pianos and top-of-the-line uprights. But you can have it custom built when you order a piano.