[sdiy] Ken Stone VCS -> confusion

[ghetto synth]

Hi All,

Between my limited electronics skills, the confusion of the LM3900,
and the overall busyness of the circuit, for the life of me I can't
figure out what's going on in Ken Stone's VCS.

http://www.cgs.synth.net/modules/pic/schem_cgs75_vcs.gif

Typically when I'm at this point I just go breadboard up some pieces
of it until it clicks but my shop is out of commission right now.
Could anyone describe the circuit operation in terms of basic building
blocks given the two use cases of 1) a trigger input, and 2) a gate
signal applied to the input. In particular, what's going on with the
3900s, the TL072 on the input and the 2N3906 just to the right of the
input TL072?

tnx