Its a $.05 Radio Shack resistor that falsifies the voltage sent from the IAT (Intake Air Temperature) sensor causing the ECM to think that the ambient air temp is lower than it really is. This in turn causes the ECU to richen the air/fuel mixture. You'd never tell a difference.
as stated basically it alters the voltage to the pcm via the IAT sensor. But i'm pretty sure it leans out the a/f. It has been proven not to work and just a waste of $$, those who think their car gained HP are all under the placebo effect.