The way it was explained to me at the factory, was that normally you use the OAT to determine your MP limit, but once you use carb heat, then you use carb temp to determine what your limit is since the engine doesn't know the difference. For example, say your OAT is 20C, but with carb heat, the air entering the engine is now 30C, you would now use the MP limit for 30C and that is the additional power they are referring to.