I think the theory is that this would come from the hydrogen enabling more complete combustion of the primary fuel.
I dont get that either. A car works by burning hydrocarbons ( petrol) in oxygen sucked in from the atmosphere. Replacing some of that oxygen volume with hydrogen would leave less to burn the petrol and worse combustion. Maybe if he threw away the hydrogen and added the oxygen instead - that would be the equivalent to turbo / supercharging in that more O2 would be in there to burn any available fuel.
