Normally when the body burns up all the carbs it can, your body will start burning fat, but can also burn protein.
Protein also accelerates the rebuilding/healing of muscles, which means less or no muscle soreness.
I think there is some controversy if the protein added really helps DURING the effort--but I don't think there is doubt that it helps greatly in recovery AFTER the ride.
Accelerade is a little to expensive for my tastes. I've been adding whey protein to my Gatorade. For me it seems to work better than Gatorade alone & as well as Eccelerade. I stick to the 4:1 carb to protien mix.