it's not. what it does is this: it takes a power-robbing belt out of the picture. the fan belt is run off the crank pulley and when the clutch kicks in, it makes the fan difficult to turn and therefore robs some of the HP directly from the crank. an electric fan only robs electricity from the battery. with an electric fan, you never get that variation in HP from when the clutch kicks in. the consensus is that that usually robs about 10 HP from the crank, but my opinion is that that probably varies greatly with altitude and age of the pulley bearings and all that.