Under lack of the necessary carbohydrates the body finds it difficult to decompose the fats. The process of breaking down fats requires the presence not only of fats but of carbohydrates as well. If the body lacks the necessary carbohydrates, the process cannot be completed and the adipose molecules cannot be fully decomposed. They are called ketonic bodies and the condition is called ketosis.  
A diet high in proteins and low in carbohydrates was popular in the USA in the 1960s. Every few decades this diet falls into favor as it seems to work. Especially with bodybuilders.
When you follow this diet:
1. You should take in a lot of foods high in proteins so as not to feel exhausted. Eggs are usually recommended in this diet as well as chicken, other kinds of meat, even milk, cheese and butter. All of them contain little or no carbohydrates, but a lot of fat.
2. You lose a considerable part of your weight during the first week.
3. Your appetite becomes smaller. This is due to the changed biochemistry of the body under this diet.

But, as I already mentioned above, this diet only seems to work. In fact what happens is:
1. As the foods high in proteins contain a lot of fat, you take more calories than you need.
2. The kilograms that you lose are not only fats. As the carbohydrate levels go down you become dehydrated and lose a lot of water (the greatest part of which from the muscle cells). Then, due to the lack of carbohydrates, your body begins to decompose muscle tissue as much as fats. So much that for every 2 kilos of lost weight after dehydration one is muscle. This is not a good strategy for a bodybuilder.
3. Your appetite becomes smaller due to the presence of adipose ketonic bodies in your blood, which in turn is due to the impossibility to break down the fats fully without carbohydrates. As a result you do not have enough energy and it is possible to feel dizziness, nausea and a long list of similar symptoms.
4. When you are working out, you will quickly spend the carbohydrate accumulated as glycogen in the muscles, use up your energy, and the lack of glycogen will dehydrate your muscles and they will look smaller. On the whole, a diet based on starving (carbohydrate starving) is not a good idea neither for an adult, nor for a competing bodybuilder.