I bought into this idea a few years back. The essence is utilizing cardiovascular activity in a fasted state to increase fat metabolism. I think its readily accepted that muscular development under a fasted state is contradictory and irrelevant, but aerobic training is a different story. Training the aerobic system in a glycogen depleted stated increases the human body’s ability to process fat as fuel v. sugar as the primary source.
Quick chemistry- glucose equals sugar v. fats equal lipids. Loads of science supports the concept that a typical American man could live for like 30+ days without food other than water based on fat reserves. Fat offers a plethora of calories …when the body learns to process it.
I went form being terrified of 10 mile jog/runs and planning water and food to running 10 miles with a half L of water and no food after 12 hours fasted. Humans survived when other animals did not because of fat metabolism.