As H2 is the ultimate combustible, its addition to the combustion process enhances the burning of the other fuels it is not only via the normal ,,chemical” burning process  but as the pressures and temperatures rise in the combustion phase H2 is transformed to a plasma state and in this state it starts to react as a catalyst for the usual fuel burning process this is why only a little amount of H2 is needed compared to the rest of the fuel/oxigen mixture.In the plasma phase H+ ions make sure the fuel is burned very efficient. this way it reduces all the environmental minus points of whichever kind of fuel you are required to use in your engine