The Parsec Harmonic Enhancer is a based on the Systech Harmonic Energizer, a filter/EQ pedal from the mid-1970s that was notably used by Frank Zappa, Joe Walsh, Les Claypool, Greg Lake and Adrian Belew among others.
The Harmonic Energizer uses a fairly straightforward state-variable filter to achieve its frequency boost. This type of filter was more often found in synthesizers and related gear, so it was an innovative idea at the time to use the concept in a guitar pedal.
While the circuit has no intentional clipping stage, the extreme frequency boost can overload the final op-amp for a bit of fuzz at certain settings.
The Parsec is a direct clone of the Harmonic Energizer, with the addition of a volume control at the end to let you dial down to unity volume at higher bandwidth & gain settings.