To break this down, lets look at a few things.
1. Glycogen Storage. There is some conflicting science out there on Glycogen storage, and like calories stored/burned, protein synthesis and many other things, we can't treat them like a simple equation. However, we can use some rough estimates. According to this link, Glycogen Storage & Athletic Performance, the average person can store 400 grams in muscle tissue and another 100 grams in the liver. The more muscle mass, the greater capacity of Glycogen storage, which makes sense given that it takes excess water to store each additional gram of Glycogen. This is why low-carb generally produces a loss of water weight up front.
2. Glycogen Starting Point, Depletion & Refilling. Again, we can't be exact like a simple math equation, but for purpose of explanation, lets just assume you're average, with the numbers above (400g muscle + 100g liver, 500g maximum total Glycogen storage). The liver uses Glycogen to maintain blood sugar levels, and the muscle Glycogen is going to be depleted during exercise. You have determine here where you are starting. If you're starting at 500g and intake is 200-300 grams of carbohydrate per day, you're going to see a slow drop in starting point. The average replenishment of Glycogen to max out in a 24-hour period is consumption of 25 grams of Glucose per hour, or 600 grams per day. Lets assume you're getting 300 per day, if the replenishment rate stays consistent, you're going to only replenish half of this after each day. This is going to be even greater if you're around the 200 gram/day point, and even lower depending on how much of that 200 grams is from Fructose. Fructose will ONLY contribute to replenishing liver Glycogen, which as we stated, is capped at 100 grams.
3. Activity Level. The above-mentioned calculation is NOT factoring in exercise. If you're doing high-intensity cardio such as sprinting or something of that nature, you will deplete Glycogen rapidly. If your activity is strength-based, you will be using less Glycogen, but even with this, some will still be lost over the next 24-48 hours. If high-amounts of ONLY protein is consumed after a workout, which is standard for most strength athletes, protein synthesis is activated with the loss of muscle Glycogenwhen it is broken down. Obviously maximum Glycogen would be lost during a very long steady-state cardio session.
4. Metabolic Flexibility. This is a hot topic. I've posted a lot about it lately. It basically comes down to how quickly and efficiently one is able to switch energy sources. Some people get become very flexible and get into Ketosis rather easily. For others, it takes longer.
Lots to consider here, but I think it's fair to say that after a few cycles of this and very low starting Glycogen levels, you're going to have the ability to become very flexible and quite possibly get into Ketosis within that fasting time of 21 hours. If you are doing the 200-300 grams per day of carbohydrate, I'd see it to be very unlikely that you really ever tap much into your stored body fat, but I DO think you can get some of the benefits of Ketosis by doing something like this. I think by maybe doing it every other day, you may even have more control over whatever results you might want.