My understanding is there is enough hydrogen (and gaseous oxygen) in the conversion (and they use Lye as a catalyst) to burn off buildup in the engine where eventually your fuel consumption goes down by a certain percentage. The claim also is that the engines computer adjusts your fuel mixture and all of this takes about two or three weeks to occur depending on the engine. The unit requires about 10 amps to run but that is adjusted by the amount of Lye in the chamber. It is possible to run it high enough to load your alternator down and cancel whatever saving your doing. The units truely do produce the gases they say. Whether they are enough to do anything productive is in question.
One can build one of these units versus buying one by using carbon rods out of an old dry cell, some pvc piping and some fittings. The cheapest one for sale is similar and the costlier ones are stainless steel.
My preference is chemically stored hydrogen which the engione runs on exclusively. We're talking 10k for a system like that once the government says we can have have it (for purchase). Then there is the cost of getting the hydrogen into the stored form which is more cost. All of this could be made cheap is they wanted it to be. They just don't want it to be. We've come a long way since the Wright brothers in planes and spaceships, but we still have clunky engines in our cars running on fossil fuels.
eh, my .02