An Envelope follower is the module used to interface non voltage emitting instruments to a CV destination (midi not included)
It will create a variable voltage that corresponds to the strentgh of the audio input signal. So the audio generated by any instrument must be electrified and sent to an envelope follower. An envelope follower is similar to an envelope generator in the respect that as your input signal will have some Attack Decay Sustain and Release that is measurable, the Env follower will "follow" the natural ADSR of your input signal rather than generating one. From this point, that voltage can be sent to any destination from cutoff frequencies, to pitch (as in the 101 and 107 respectively) to any voltage destination. Some envelope follower modules lke the synthesizer.com module has a point where a gate signal can be generated if the audio input reaches a certain level. At that point it can be used to trigger events such as the start of stop of a sequencer, or to trigger envelope generators connected to other devices. Its a great module, especially for stringed instruments or any instrument that gives you dynamic control over the volume (such as a theremin).
Heres a vid you might find interesting:http://www.youtube.com/watch?v=csZ2n66Vdas
Hope this helps you out.....