A spool is a thing, that replaces the diffcase+internal gears, it gives the same amount of power to both wheels. (it is a difflock, you just need to screw the ringgear of the diff on it (with an e-maxx))
It's a nice and cheap alternative for a low cost bulletproof diff..
Take a look at this picture, you'll understand;
It is made by traxxas and costs about 10 bucks..