Alan Dudley replies to Jeremy McMillan:

>I believe, but am not certain, that the speed at which the key descends
>is measured and controls the volume of the sound.  It could also modify
>the length of the attack part of the sound, or the ratio between the volume
>of the attack part and the continuing part.

All true.  In a MIDI keyboard controller, this measurement is called
"velocity", and the synthesis engine can alter various parameters as a
function of this value.  The most common are, as you note, the "envelope"
parameters that are used to control the amplitude of the sound, the filter
cutoff frequency (which controls the "brightness" of the sound), possibly
the oscillator frequency (which controls the pitch), and sometimes the
waveform shape (which will affect the harmonic content of the sound).

However, I think Jeremy was asking about how the key works mechanically to
reproduce the feel of a piano keyboard.  In the best keyboards, the key is
weighted/balanced to reproduce the dynamics of the piano action.

len.