Originally Posted by Varius

In brief:

The body burns glycogen (from carbs) for fuel. By restricting the source of glycogen (carbs), once your body has depleted its stores, the next place the body turns to for fuel is fat.

So, by keeping carbs at ultra-low levels (I personally choose to only consume 5g - 10g a day) and providing enough fat, you prevent glycogen build-up and induce an awesome fat-burning state called ketosis for very extended periods of time (some cultures in the past did this for their entire lives).