[i]"I'm not sure exactly what is happening with the volume either - in the first few notes from 3:10 I'm only switching the volume - the pitch is as steady as a theremin pitch ever is - I had been guessing it was an amplitude modulation thing, but it could be anything." - Gordon [/i]
Had another look / listen (no, I didnt do an analysis of the waveform - there is so much else going on that it wouldnt be worth the effort).. I believe there is just a tiny bit of ground 'loading' causing the effect.. I got muddled in my thinking, and equated increased amplitude with increased capacitance (touching the knife) - Of course, its the other way 'round! Duh!
What I believe is happening is that, when touching the blade, your body's increased coupling to ground (via loading from the volume antenna, EW circuitry, or other grounded objects) raises the pitch fractionally, and when this contact is broken, the coupling decreases, so the pitch lowers fractionally.. Its probably less than 10c, but enough to add a nice 'extra' to the sound.
Fred.