From NonLinear Circuits website:
This is a simple circuit that is capable of making a wide variety of sounds. With a sequencer it can be used as a crude VCO, but fed other signals, such as from envelope generators and LFOs, it will sound like anything from a baboon with the squirts to a cheesy 80s video game laser cannon.
The name is drawn from the two main components that define the function and characteristics of the module. One is the vactrol which is a light controlled resistor, these have an interesting effect whereas there is always a time delay whilst the resistor changes its value – this is a good thing and has been exploited by many synth designers, notably Don Buchla. The PiLL is from a PLL – phase locked loop, this is a special purpose IC which contains number of subcircuits and usually used in radio and digital control circuits. It contains a VCO, logic gate and a phase detector = all fun stuff.