Fat oxidation is the breakdown of triglycerides stored in fat cells, and free fatty acids in the blood for energy. During any activity there will be some usage of fats as energy. Specific types of exercising that use fat as a primary source are intervals. That is when the physical exertion alternates between high and low intensities. Fat is also one of the primary energy sources during rest. That does NOT mean that being a couch potato all day will make you burn a ton of fat off.