From NonLinear Circuits website:
This module is an audio rate chaotic signal generator; it contains 3 phase locked loop ICs (PLL) connected in a circle via their phase difference inputs.
It was inspired by the paper - A New Type of Intermittency from a Ring of Four Coupled Phase-Locked Loops by Hasegawa et al – although uses just 3 stages and has external control.
The circuit uses vactrols and CV inputs to control each PLL. There are 6 inputs; the upper two inputs are connected to the switching pins of the lower 4 inputs. This does not mean the 6 outputs will be the same, as each stage is phase-modulating the next it means the outputs are all different, sometimes a little and sometimes a lot.