a spool permenantly locks both axles together so that both wheels get exactly the same power at all times. when turning, a spool will cause one wheel to drag on the ground a bit and make scraping sounds.
a locker gives equal power to both wheels just like a spool but when on the street, it disengages when not on the throttle so you can turn without the tire wear and noise.
i believe the mini spools arent for drag racing. the poor man's way of making a spool is just welding the differencial gears in the trac loc you already have so that they cant spin.