HIIT is based on the phenomenon of EPOC, or Excess Post-exercise Oxygen Consumption. It alternates short bursts of high intensity exercise performed at the peak of the athlete's performance with short periods of lower intensity exercise. The high intensity exercise is designed to use up as much of the body's store of resources as possible.
As a result, the body starts using all its resources and oxygen. Once the oxygen supply is diminished, the individual can sustain exercise only for a few more seconds. This is anaerobic exercise that creates an "oxygen debt". It is this "oxygen debt" that encourages the body to use up excess oxygen after the entire workout is over.
This exercise puts a strain on the body's resources and causes the body systems to function at a more stressful level. After the workout, the body tries to return to a resting state, also known as homeostasis. To do this, it uses up all the existing resources in the body. Since the intense workout has already stripped the body of most of its resources, it relies on the glucose and the fat reserves to perform the functions required to return to normal.
High intensity exercise also stresses the muscles of the body. This kind of exercise creates micro-tears in the muscle fibers. Muscles are huge consumers of oxygen. Repairing them also requires energy. The only energy source that the body can use are the existing fat and glucose reserves.
It may take up to a day to return to normal after a HIIT workout. EPOC, or the afterburn effect, is active throughout this period. It is much more effective and consumes many more calories than it does after a few hours. It is this afterburn effect that leads to the loss of calories and fat even after the HIIT workout.
HIIT can be performed with both, aerobic exercises and anaerobic exercises. The difference between the two forms of exercise lies primarily in the fact that aerobic exercise mostly burns fat and calories during the exercise, whereas anaerobic burns calories after the exercise.
The loss of calories during normal aerobic exercise as compared to normal anaerobic exercise was almost the same, as the calories burned during the aerobic exercise of normal intensity. However, the calories burnt after anaerobic exercise of high intensity were found to be much higher than the calorie burn during aerobic exercise of equal intensity, according to a Purdue study from 1992.
Thus, HIIT with anaerobic exercise will produce more results than HIIT with aerobic exercise.