You might be surprised how little research there is on fat adaptation, but I'll do my best to fill in the gaps with informed speculation.
With low carbs following a high carb diet, the body will first attempt to use sugars from your glycogen. Your glycogen will gradually deplete. When your glycogen depletes entirely, you will enter ketosis.
As you adapt to fat burning, your tissues will stop demanding sugar so urgently and will start to accept fat more readily. As a result, carbs over 100g (the amount demanded by the brain and a few other tissues) will begin to refill glycogen stores. Additionally, excess protein will be used to refill them.