@GenevaMech- To be honest with you, your body burns more fat when you have depleted your muscle and liver glycogen stores. When you have reached maximum glycogen breakdown, your body can actually burn up to 300% more fat according to different studies. Now whether or not you can sustain any sort of workout beyond that point is where the question lies.
Glycogen is essentially stored carbohydrates, so once the body depletes these reserves, than the body switches to burning stored fat. Once you "hit the wall”, a little rest and light exercise will allow the body to transition into fat burning mode. At this point, you should be able to continue exercise that will burn more fat calories.
This is the reasoning behind the theory of working out before you eat anything in the morning. Your body has essentially fasted overnight, so its glycogen stores are naturally depleted. You will burn more fat calories during your pre-breakfast work out than if you work out after you have carb-loaded.