That is either faking out the MAF (Mass Air Flow) or IAT (Intake Air Temperature) value to the computer. On a gas car, changing the MAF value can cause the vehicle to run rich or lean, and running lean increases gas mileage (at some mild risk to the motor, depending on how lean). On a diesel, it just causes either more smoke on spool-up, or a slower spool up, depending on what is done to the signal. It will not affect mileage or peak power at all. It
might cause slightly better throttle response at the expense of black smoke.
If you want more power, and potentially better mileage, you need either a tune (EFILive, PPE, etc) or a plug-in module like the Edge. It is usually best to just get a tune (EFILive), the Edge box isn't that great, IMO (and I used to run one).
Edit:
This is probably what he bought ($65!).
Customize your performance module:
Note: If your vehicle's information is not available to select, please purchase any combination and enter your vehicles information in the "Comments" during your checkout. We will build the module for your specific vehicle.
Yeah, no.
Randy, you need to watch this:
[YOUTUBE]VGtImIP6j3A[/YOUTUBE]