Car engines have complex processes to which end they produce mechanical energy. This poses a number of problems/oportunities to the public and commercial sector - in terms of fuel, energy storage, collection, efficiency and perhaps a lot more aspects. I believe this avenue should be explored rather than shooting it down as "crack pottery" which I feel is simply the assumption of those who do not bother to look further into ideas that challenge their idea of science. This is exactly the reason why energy production has stayed in fossil fuels for so long, and technology is actively supressed.
That said, I believe the performance gain in these modifications is not due to the explosive effect that hydrogen has on the piston in the engine cylinder. To think that would be silly of course, if you understand the fundementals of thermodynamics and energy. Rather it is a catalytic effect that the fast burning hydrogen/oxygen mixture has on the fuel air mixture, causing higher fuel burn ratios - resulting in more power.
But it doesn't stop there - this needs to be more thoroughly researched and tested on a variety of engines before we can really see where it has the most benefit. Metals, electrode designs and configurations have to be further researched. Steel has already proven unsuitable for use in cars, although titanium fits this application remarkably from my research. Then the correct amount needs to be identified for peak performance to specific motors. It's something car manufacturers could do easily, but they have no desire to until we really push them.
The benefit to being able to store hydrogen in a cylinder (pure hydrogen, not hydrogen-oxygen mixture) is that it can be safely transported under pressure, much like a battery is a standard part of a car, even though its full of acid and poisonous metals. If hydrogen can be cleanly separated from these devices, it wouldn't matter if it costs you more energy to produce it - it could be made from solar or other free energy readily. The fact is you're trying to have condensed portable energy that could compare with camping gas, or even petrol with efficient designs.
You see, there's usually something to these crackpot theories. It's just that linear science in its vastly limited understanding (and for all its usefulness), gets the better of most people. I guess most people like to follow the big guys and not think or experiment for themselves. :-/