Claimed theory
For a start the ECU has voltage regulators in it to give sensitive electronic devices a smooth voltage supply. Other things like injectors/soleniods, motors and coils are inductive devices and dont care what voltage ripples they get.Because of many reasons, a standard engine never combusts the fuel completely. Magic Booster solves this problem by improving the ignition (gas, lp) and fuel injection (diesel). The alternator doesn't generate steady electricity (see the white line in the diagram). Also, the other electric appliances inside the car share electricity at the same time. Therefore the fuel cannot be completely combusted due to unstable ignition spark(gas, lpg) or fuel injection (diesel). Because Magic Booster covers the gaps between the crests while it detects the weakness of electricity (see the purple line in the diagram), resulting in steady & complete combustion that helps to make your engine run smoother and save the mileage.
I wonder how many they sell? Same goes for those electric superchargers and grounding kits etc.
Its a wonder eBay cant have them removed for false advertising