"Clinical Ketosis Diet" to be precise. Where you carb-deplete(10-30grams) from Sunday to Friday, then carb-load for 36 hours.
Purpose being, is that if your body doesn't have a its' primary source of fuel to burn(carbs), because you are severely limiting the amount you intake, that it will have to revert to another source for a fuel supply. Ergo, fat.
Anyhow, it is tougher than it sounds. You can eat more, but there is a very limited variety of what you can eat. Most foods that Americans eat are chalk full of carbs.