Note: This is not an overdrive or distortion pedal. It is a fuzz pedal! Different beast.
Unlike passive bass and guitar tone controls, the tone control increases high end when turned up. The "bass boost" switch selection adds bottom end back in; this can provide the bass version of scooped mids, or just put some bottom end back on your fuzzy slap.
The "dry" position on the switch mixes a set amount of your dry signal with the distorted output, which lets you tame the fuzz somewhat.
If you want a fuzz for your bass, if you like the sound of a Big Muff Pi for guitar, this is a great deal for your bass.
Electro-harmonix Bass Big Muff Pi
Big Muff Pi optimized for bass. "The Prodigy reborn with underworld instincts comes to life wielding the drive of the original classic Big Muff Pi