Preserving Protein
BCAAs offer a definite protein-sparing action in muscle during low-carbohydrate dieting. When glycogen stores in muscle and the liver are low as a result of limited carb intake, the body tends to tap into muscle aminos as a source of energy. That's more likely to happen when total calories are very low in relation to exercise or when body fat is below a certain point.
The leaner you are, the higher the risk of tapping into muscle amino acid stores, especially during aerobic exercise. Taking BCAAs before cardio spares - that is, doesn't gobble up-muscle aminos, thus preventing excess muscle protein breakdown during training.