It is doing a cardiovascular workout first thing in the morning on an empty stomach. Usually for about 45 minutes. The idea behind doing so is that your body's glucose levels are low after the previous nights fast. So, in theory you will use your body fat as the source of fuel, therefor getting leaner, faster. Most people who are attempting to lose weight employ strategies that make it literally impossible to preserve muscle. Now, fasted cardio actually works, if you're using PED's (Performance Enhancing Drugs). The anabolic aid is the only way to protect your muscle mass, otherwise, for us Natural Athletes, fasted cardio is a sure fire way to eat at our muscle. Thus losing our GAINS.
So with fasted cardio in the mornings, your cortisol level is going to be very high. Actually it's at the highest in the morning. Cortisol is a muscle wasting hormone. So it's already elevated, and now you introduce cardio which tends to raise cortisol output. If your cortisol level gets too high you'll have a hard time lowering it, and spend the entire day in a muscle wasting state. This is ten fold when in a caloric deficit.
The ideal way to achieve the most caloric expenditure from cardio is to do it during what's called the 'post-absorptive state'. Ok so what does that mean? Post-absorptive state is when you've eaten, but no longer digesting your food. It is when your bloodstream is nutrient heavy and fat oxidation is at its height. How do you know when that is? With solid food it's very difficult to know, as it is relative to the person. Using a supplement, like Supplement Rx's Building Blocks Protein or Rx Meal, the body's absorption rate is much faster than that of solid food. So taking these products 30-45mins before an exercise bout will keep you out of the catabolic state and in that post-absorptive state. Therefor maximizing body fat loss!