Great little boost pedal, good for the single coil guitars to give them a little more oomf! Heres what seymour duncan says:

application
The first effects pedal designed specifically to work together with the pickups in a guitar to achieve optimum gain and resonance for all tonal applications. Works with all musical styles.

description
By turning the gain control knob you can add pure, flat-EQ gain, ranging from 6dB to 25dB! The Resonance Switch drops the resonant peak of a single coil pickup up to 5KHz, making a single coil sound like a full and fat humbucker. Features fully discrete Class A circuitry along with true bypass. Comes in a heavy-duty steel chassis and runs on a single 9 volt battery.

of note
For Resonance Switch to work properly, the SFX-01 must be first in the signal chain using passive pickups and a standard guitar cable — not a wireless unit.

players
Michael Landau, Peter Stroud & Todd Wolfe / Sheryl Crow Band, Richard Evans & David Rhodes / Peter Gabriel, Gary Rossington, Rickey Medlocke & Hughie Thomassen / Lynryd Skynyrd, Greg Hetson / Bad Religion, Phil Campbell / Motorhead