Modifying a hardware component to feed it more or less voltage. Particularly zealous overclockers may overvolt components to allow them to run at higher clockspeeds. Particularly zealous efficiency nuts may undervolt components to trade decreased performance for decreased power consumption.
In both cases, part of the trade typically also is reliability. The farther you go outside of the spec for parts, the higher the risk that they work only sort-of. I gues that, if money wasn't a problem for this guy, he would have bought ten different CPUs and checked which one kept working at the lowest power.
