From the marketing blurb:
What the Eruption pedal aims at is to achieve the brown sound through the Variac control, just like in the Plexi amp used by Eddie Van Halen*, with the possibility to manage the virtual voltage through this simple control which, if kept at minimum, the character remains the same of a stock Plexi. As this particular control is raised, the same frequencies of an under-powered amp are reduced to emulate the physical phenomenon that reproduce the Variac.
..... well I HIGHLY doubt all of this.