As I understand it from reading online, pumping the HHO gas in the engine makes the computer think it's running lean because of excessive oxygen in the exhaust. So it enrichens the mixture. Now it there is still some mileage difference without this gizmo, but with it the results are much better.
You calibrate the Oxygen sensor control unit, by reading the voltages at full warm idle and you average the readings, then you attach the gizmo in between the O2's and the ECU, and tune it back to those average numbers with the HHO feeding into the engine.
It does work because I was only averaging about 50-60KM / tank without this device and just HHO and with both I get 200KM extra per tank.
And yes, I have access to a gas analyzer emission machine at a friends auto-shop and have played with the settings both with/without HHO and both with/without the O2-sensor gizmo to see how things are affected by what settings.
Now I'm not running the HHO unit, but I do have the O2-gizmo connected and leaned out a tad to save me fuel, but not enough to increase the NOX. I'm saving about 30-35KM/tank with this setting so I have left it for now.
I haven't thought about talking about it here for fear of people tearing apart my "for hobby" experiment.