The body utilizes fat as a fuel source in this nutrition system. When excessive carbohydrates are consumed the body utilizes this as a fuel source through releasing insulin to transport the carbohydrates to the muscles to burn. When your intake is mostly protein, fiber and fats then your body will begin to use more fats for energy. There are four phases in this system, it begins with few carbohydrates to transition your body over and slowly adds them back as you progress.
Dr. Robert Atkins created the Atkins system. If you would like more information, check out http://www.atkins.com/Home.aspx