The Shoosh is a vactrol-based voltage controlled amplifier (VCA). It's basically a volume control that can be set using a reference voltage from 0 to 5 volts, so this pedal can effectively fill the role of a tremolo, compressor, sidechain or volume pedal depending on what's controlling it.
The single knob controls the total amount of gain.
The board was quite small and only a single knob was required to control it, so I built it into a 1590A sized enclosure. Even then it was a tight squeeze that required the vero layout to be planned out with a little care.
The circuit is an original design, although not an original concept by any means. A vactrol is used to turn an externally provided control voltage (CV) into a resistance, which then controls the gain of the output. For much more information on how the circuit works and how to build it see my post on "building a vactrol-based VCA".