Not my understanding of the science behind it, but perhaps I am wrong.
My understanding is that your body uses carbs for energy first, and when there are no carbs available to burn, it gets energy from your fat store, thereby reducing fat.
Conversely, your body will take excess carbs that it could not use for energy and store them as fat.
It takes more energy to break them down in your stomach than carbs do. Fats and proteins require more energy to break down than carbs. Thats why you burn more energy eating them than the energy heeded to break down carbs, which are easily converted to glucose.