The best "explanation" I've read of why water4gas works at all is that it is
basically the equivalent of adding a water injection system to your vehicle.
The water increases the octane of the fuel because it slows combustion.
Unfortunately, water injection will also substantially decrease the lifespan
of your engine.

The miniscule amount of hydrogen that is produced has a negligible effect on
your engine's performance.

One more thing... the modification of your vehicle's emissions system is
illegal in most states.  Not that they're going to hunt you down or
anything.

Jeff