Actually the temperature is controlled by the flow rate through the radiator. The thermostat opens as the temperature climbs, once fully open it cannot allow anymore flow than what the water pump provides. The radiator is sized to handle the maximum expected heat load of the motor. Therefore to run cooler either get a larger thermostat opening or lower the viscosity (resitance to flow) of the coolant or improve the heat transfer coefficient of the coolant. I would suspect the additive reduces the viscosity so that flow is improved with a smaller thermostat opening. The radiator is simply a heat exchanger with a fixed volume. The mass flow rate increasing through the radiator will exchange more heat since more heat is available to be exchanged. The standard equation is mdot (mass flow rate) times difference in enthalpy (delta h). This takes advantage of the first law of thermodynamics (heat can only flow from a higher source to a lower source), so the equation can be rewritten mdot times delta T (differential temperature).
Ever notice that engine coolant is kinda thick compared to water? That additive is simply an adjuvant that makes the coolant flow better (makes water wetter).
