The Blueprint (and the Soviets) owe a deep debt of gratitute to the founder of GAS theory, Dr. Hans Seyle. In 1926, he formulated the General Adaptation to Stress theory consisting of 3 phases: 1.) Alarm 2.) Resistance/Adaptation and 3.) Exhaustion.
It was from this framework that the coupling of training, diet and adaptogens was refined. Biochemically speaking, here's what's going on inside of you during the Famine phase:
The Hypothalamus-Pituitary-Adrenal System
This system governs the amount and kind of response the body produces to combat a stressful environment. Simplified, the hypothalamus (the bridge between the brain and endocrine system) sends a message to the pituitary gland (a hormone-producing gland embedded in bones at the base of the skull) to release ACTH (adrenocorticotrophic hormone) into the blood stream. This signal prompts the adrenal cortex (located above the kidneys) to create corticoids, another hormone, from available raw material. These corticoids are then dispersed to the places in the body they are needed, where they are put to use in the various stages of defense against a stressing agent (your workouts and calorie/protein depravation diet found in the famine phase).
Obviously, you don't want to spend too long in this phase, which is why I recommend just 5-7 days be spent here. Done correctly, the alarm signal is properly delivered with minimal loss of lean muscle mass, setting the table for a long run of adaptation/hyper-growth seen in the feast phase.
Tip: Remember to guage increasing stress during this time by taking your waking morning heart rate (beats per minute). Few of us have the time/money for blood work. Resting heart rate is an fast, objective measure of alarm status and you should see it rising during this time. You can also validate success here by measuring protein breakdown via Multistix urinary testing strips.