What hes probably talking about is something like the "Tornado" and they are all crap. The theory is that you put this device into air intake system for the car and it swirls that air in this magic way to be more efficent and thus increaseing your MPG and power.

Simply put the only way to increase your MPG is to drive conservatively, proper air pressure in tires, no jack rabbit starts or fast stops (basically when approaching a light try and coast if possible), lighten the vehicle if possible, and dont drive too fast.

Tim