Drive: Sets the preamp level going in to the fuzz section.
Screech: Adjusts the feedback loop level when the “screech” switch is activated.
Bias: Controls the bias point of a pair of transistors.  All the way up, it produces a normal fuzz tone.  All the way down, a slow attack is produced.  Somewhere near the middle produces the most upper octave.
Tone/Tone Bypass: Tone control.  When bypassed, produces a volume boost.
Level: Output volume.
Exp Pedal: An expression pedal can be used to control feedback oscillations when in screech mode.