With the lightest diff fluid Power will be mostly delivered to the wheel with least resistance and that tire will slip. Applying heavier diff fluid will counteract this and will allow power to be delivered to both wheels much more equally. Think of it as light weigh fluid is a vehicle with an open differential in a 1:1 vehicle that will do one wheel peels and heavy fluid is a 1:1 vehicle with a locker that will roast both tires.