Energy is stored as glycogen in your liver and muscles, which is glucose combined with water. When you start the diet, the first thing your body does is burn up your glycogen, releasing all that water. That is where it gets the energy you need when you stop eating sugar/starch. The glycogen is heavy, so you will show a dramatic weight loss as you burn it. You will pee about 3-4 litres extra in those first few days. I was getting up 4 times a night for the first 3 nights.
When the glycogen is depleted, your body will start to burn fat. This is when the real weight loss happens.
You may lose around 8 pounds of water the first week, then settle into about 2 pounds of fat loss a week. If you don’t revert to your previous eating habits, this fat loss will be permanent.
After 8 weeks, you will have lost over 20 pounds, about 8 pounds of water and 12 pounds of fat. Every extra week will mean an extra 2 pounds of fat lost.
If you fall off the wagon and have a high carb day, the only thing you will do is rebuild your glycogen levels. You can easily gain 3 pounds in 1 day, but this is just water. A couple of 800 calorie days and those 3 pounds will quickly disappear.
When you get to your target weight and up your calorie intake, you will rebuild your glycogen stores so you will regain the water you lost in week 1, which is fine and expected. You should overshoot your target weight to account for this.
At that point you need to go on a good exercise program to help prevent regaining the fat you lost. A tape measure is your friend as weight gained by building muscle is good, but you want to keep the fat off your gut!