What is the Theory behind Interval Training?

The theory behind interval training is as follows:

It is a myth that low intensity cardio training is best for fat loss just because more fat is burned for fuel as a percentage of the total calories burned.

When you exercise, you are always burning a combination of fat and carbohydrates (in the form of glycogen - the stored form of carbohydrates). This ratio varies depending on the type of training you do.

Low Intensity (L.I. for short) burns about 50% fat for fuel while High Intensity (H.I.) burns about 40%. This is not a big difference.

Say, for example, you burn 100 calories in 20 minutes of L.I. work compared to 160 calories in 10 minutes of HI work, you've still burned more total fat doing HI.

Low Intensity
100 calories x 50% = 50 calories

High Intensity
160 calories x 40% = 64 calories

High intensity training will also boost your metabolism long AFTER the workout is done due to the increased demand on your cardiovascular and muscular system. This does not happen with low intensity training. This is the reason it is said that interval training burns fat all day. Your body will also be burning carbs at the same time, of course, but since your metabolism has been raised, you will be burning more fat too.

The basic idea when you're trying to lose fat is to create a caloric deficit. The type of training does not matter as much as creating that deficit. High Intensity training just creates the deficit more efficiently than Low Intensity training.