OK, this is geting complex, but has potential.

The gas, being complete with it's own oxygen, does not need to be mixed or calibrated & in modest doses should not affect the air/fuel ratio of the intake of petrol/gasoline.
It is used to suplement the gasoline component of the engines operation, but here's where it gets a little complex.

Production of the gas is greatest when manifold vacuum is highest, such as in light throttle cruise or when decelerating. This is when we don't want large volumes of gas.

Gas production is lowest when manifold vacuum is lowest, such as when towing up hills. This is when we want it the most, so vacuum is working against us.

This might be overcome with some kind of throttle linked potentiometer that ramps up the voltage at high throttle settings & shuts it down, perhaps even off at idle.
The multiplate electrodes [like plates in a battery] seem to have the greatest potential to produce large volumes of gas.

The more I learn the more complex a successful working system becomes, but success is achieveable as long as it is used as a suplement.
Since the gas adds combustion pressure when it is used it means that less throttle can be used to provide the same power output as before & the fuel savings are achieved that way but I don't know about a system where the engine runs on HHO alone.
Anyway, the whole thing has potential for those who are prepared to spend the time & some cash to install a properly engineered ststem but the simplistic systems that have been used for demonstration purposes so far don't come close to being a success in the long term.