Originally Posted by digitrc
A spool is a crutch for when you can't stop a diff from unloading... there's the science!
Diffs unload because of weight transfer. There is more weight on the outside edge of a cornering car.
Wouldn't it be nice if the outside wheel was putting down power if the inside wheel had less weight like mid and corner exit? The answer is a spool (or one-way for the same reason while you are on power but slightly different as well)
There are some cases where a diff is much faster, usually high speed chicanes where the car has to switch quickly.