If you were out in the desert and you had no food, for the first 24
hours, your liver would break down the sugars/carbohydrates it had stored away. Then, it will turn to breaking down your stored fat and protein (in the form of muscle) to keep your blood ketone (a product of fat metabolism that the brain can use instead of glucose/sugar) levels high enough for your brain to function. At the same time, it will tell the rest of your cells (which it considers less important) to reduce their metabolism so that there is enough energy to go around for your brain and heart. This is called starvation mode.
When you reduce your carbohydrates to a level less than 20 grams a day, you have automatically within a period of 24 hours, shifted your body into what it sees as starvation mode. Hence, it will signal your cells (muscle, skin, kidney, etc), other than those of your brain and heart, to reduce their metabolic activity. Thus, your overall metabolism goes down.
I would love to hear your ideas.