I've got one on the derv, Rover ron's, all it does is compensates if the MAF sensor is on the way out.
Rather than buy a MAF sensor that was expensive, you plug the box in and it fools the ecu to think its all OK.
Think he has stopped selling them now as the MAF sensor is a lot cheaper so not worth doing.
It never added hp, only made the engine perform to its peak condition.
Anyway thats what I think its all about.