The Burriss Boostiest is a combination of two totally independent gain pedals in a single enclosure. On the right side (input side) is a fully adjustable clean gain pre-amp On the left side (output side) is a "TS"-flavored overdrive circuit.
Both circuits feed from the same power source, a single 9V battery or a standard "boss"-style power supply. This power source is divided and stepped up to a 16V ~ 18V operating voltage for the clean gain boost side and lowered or "browned out" for the overdrive circuit. I believe that this topography gives the best combination for a big, clean tone and a fatter overdrive tone.
Both circuits are true by-pass and can be used alone or together.