The infamous Harmonic Percolator circuit tuned for maximum tone. Capable of filthy sub octave and wilful feedback, through to just a hint of grit, or an amp slamming transistor booster. Definitely more than just a fuzz.
As in the original HP-1 the Hurts uses both germanium and silicon transistors carefully selected and graded for both sound and gain. Inside it has a couple of trimmers, one to tune in the octave "sweet spot" and one for gain compensation in case of oscillation.
It uses germanium diodes as in the original HP-1 and has a switch to change to silicon diodes which are slightly louder (my favourite on bass). In the mid position this switch bypasses the diodes making it a loud transistor booster that really pushes the front end of an amp.