Ok I'm no expert... but from what I remember... insulin plays a role in this... when insulin levels are up (like after you eat) then you are using mostly carbs for energy... as insulin levels drop you burn fat...
BUT you need carbs in your blood... you need a certain level for brain function... etc... when you first burn off the carbs in your blood... there is some stored in your liver... and when that is used up your body will consume protein to create carbs... or something close enough... (leaving out heavy duty science)...
Also it isn't black and white... like use carbs.. then when those run out... blah blah...
BUT I think what you are refering to is a precedence of what energy sources are used when you eat... excess whatever is stored... or whatever you don't burn is stored... so when you eat... your body burns (uses) the food for energy as it enters the blood... it will burn in the priority of: alcohol, carbs, protein, fat...
But when you use up what is in your blood and it needs to pull from reserves... protein is last... it will only catabolize (eat oneself) protein/muscle if it runs out of stoored carbs.
so summary...
When you eat... food enters blood... excess or what you don't burn gets stored... energy in blood used in Alcohol, carbs, protein, fat.. .order
When your blood in deficient in energy we pull from reserves...
Body needs x amount of carb energy which it pulls from reserves... primarily in liver... and when it runs out it catabolizes protein/muscle
when in this state of energy defieciency you also burn fat...
hope that helps...