These include the essential amino acids leucine, isoleucine, and valine.  They are very popular among athletes and there is some research validating their use.  Numerous research studies have shown these 3 key amino acids are extremely important to consume, especially when you are dieting and exercising (and according to one study, BCAA's are even more important when exercising in the heat). During exercise, your body uses a mix of glucose, fats, and even protein as a fuel source. When you diet and your carbohydrate intake is lower than normal, the percentage of protein your body uses for fuel (specifically Leucine, Isoleucine, and Valine) dramatically increases. The body will pull those needed amino acids from the continuously circulating pool of amino acids in your bloodstream.  And if not replenished from an outside source, i.e. specific amino acid ingestion in the form of BCAA's, your body will again, breakdown other areas of your body in order to supply this pool. Studies have shown that subjects who consume an effective dose of BCAA's while dieting have greater levels of lean muscle mass retention than controls that ingest a placebo (and typically LOSE muscle during the same dieting period). 

Results from one study concluded that the subjects consuming the high protein diet including branch chain amino acid supplements lost the greatest amount of bodyfat. Even more compelling is that the group supplementing with branch chain amino acids lost the greatest amount of fat from the abdominal and thigh regions, two areas of concern for many men and women in regards to fat loss.   It is important to take gram doses of each of the BCAA's before, during, and after exercise to maximize a workout program.