The little unit produces hydrogen, which is plumbed into the intake. ( i cant remember the ratio of hydrogen to fuel, just quite low is all i know)
One or two computer modules that play with the computer to play with the sensors, i know the older units without it did not work well at all. Before the computer would freak out and dump more fuel into the engine, thus getting worse millage.
But he seems to be making mpg gains all the time now. Pretty much all you have to do is add distilled water to the unit.
Ill try and get the website from the one he is using.
Either way its interesting.