In the words of EarthQuaker Devices:

The Special Cranker is an all-discrete analog distortion enhancement device designed to give your signal some extra grit and boost without drastically altering your tone and retaining all of the natural nuances and character of your amp. Think of it as sticking an extra preamp tube in your amplifier for more saturation. Single notes will blossom and bloom with rich harmonics and singing sustain with responsive, tube-like sag. Power chords will sound thicker and crunchier, a little more ragged around the edges. Complex chords will remain distinct and complex, as the Special Cranker is designed to preserve your tones integrity without muddying up the low-end or smearing the midrange.