A ketogenic diet supports weight management by assisting the body to burn fat, rather than carbohydrates. Ketosis occurs when the body can no longer rely on carbohydrates  for its energy source, thus it starts burning fat, in the form of Fat Ketones. 
By reducing carbohydrates to between 5% to 10% of daily caloric intake, the body will over time divert fat into ketones, thus burning them for fuel. Depending on the diet, at this level people may be consuming between 10-50 grams of carbohydrates daily. Usually a person wishing to become ketonic, will use urine test strips to measure the level of the bodys’ ketosis. 
Due the reduction of carbohydrates, protein and fat macros are increase, although this is not highly recommended, as the liver may be put under strain from excess protein, and cholesterol levels may increase due to additional fat. This is why those who practise regular fasting, often during the fast, become ketonic.