In the absence of adequate carbohydrate, the body has difficulty metabolizing fat. The process of fat breakdown requires the presence of both fat and carbohydrate. If your body lacks the necessary carbohydrate, the process cannot be completed and you end up with incompletely metabolized fat molecules in your system. These are called "ketone bodies," and being in this state is called "ketosis". The high protein, low-carb diet for weight loss was first popular in the United States as early as 1860. The concept returns to favor every few decades because it does not seem to work especially for bodybuilders. Whenever you follow this kind of diet:

You get to eat a lot of high-protein foods, so you do not feel deprived. Eggs are usually recommended on this diet, as are chicken, fish, meats and even milk, cheese, and butter, all of which contain little or no carbs but lot of fat.
You lose a significant amount of body weight within the first week.
Your appetite tends to diminish. This is due to the changed biochemistry of the body on a low-carb diet.
This kind of diet only seems to work. Actually, what is happening is that:

Because these high-protein foods contain a lot of fat, you end up with more calories than you need.
The weight loss you experience is not all fat. As the carbohydrate level in your system declines you begin to dehydrate and lose a lot of water weight (much of it from the muscle cells). Then, in the absence of carbohydrate, your body will metabolize as much muscle tissue as body fat. So every two pounds of weight loss after the initial dehydration, about one pound is muscle. Hardly a good trade-off for a bodybuilder.
Your appetite diminishes because of the presence of fatty ketone bodies in the bloodstream, owing to the body's inability to completely metabolize fat without the presence of carbohydrates. So your energy diminishes along with your appetite, and you are also liable to develop dizziness, nausea, and a whole host of other symptoms.
When you exercise, you will quickly use up the carbohydrate stored as glycogen in the muscles and will run out of energy; and the depletion of the glycogen will make your muscles shrink and look smaller.
In sum, dieting by starving yourself of carbohydrate is not a good idea, either for the average person or for the competition bodybuilder. The disadvantages of following this regimen far outweigh any advantages.