My mate bought a box to attach to his Disco 3 TDV6 diesel. He swore blind that it improved power (which it did) and reduced fuel consumption. He got a little bit annoyed at my cynicism that it could do both, showing me the better average MPG on the display.
After some research, I found out the trick - it simply fooled the engine into overfuelling slightly, making it think it was putting less fuel per stroke than it actually was. So it gave more power, but of course the MPG calculation is based on the same figures, so since the fuel measurement was off, so was the MPG.