It’s such a simple patch but does such a fascinating thing. I’m posting the image because it’s simple enough to make yourself.
When you frequency modulate an oscillator with itself, depending on the original waveform you add harmonics up to a point, then it starts bending the root pitch, then it starts turning into digital noise at the maximum modulation.
Feeding the 4 waveforms of a Fundamental VCO-1 into a Skylights Vactrol and then feeding it back into the oscillator’s FM input has the effect of making weird random slidey melodies. For a 4 module patch it generates some truly wacky output.