I've helped build one mini spool diffs, and I've driven two cars with them.
A mini spool is simply used instead of spider gears, or LSD clutches.
In a single spinner diff, there are a series of little gears that allow both axels (wheels) to spin at different speeds. (LSD uses clutches)
With a mini spool, the two axels are locked together, with the carrier.
This means that there is no slip when you drive your car around a corner. Both wheels will always spin at the same time, regardless.
The advantage of a mini spool is that they are dirt cheap, about $120 and can be installed into a stuffed single spinner or LSD diff, provided the axels are still good. Oh, and you can do some seriously crazy burnouts with these diffs.
Te downfall to this type of diff is that the car seems to hop around corners, and tight turns can become annoying. The cars cornering ability is reduced. I imagine that parallel parking would also be annoying. There is increased tyre wear from this setup, as well as more strain on all the drive train components, and things like control arms.
Also, axels my tend to twist severely under heavy loads, which could result in one being snapped. I have been in a car when an axel has snapped during a burnout, and the car was still drivable.
I'm not sure if they will past road worthy either.
So there you have it.
JUST THINK OF THE BUNOUTS!!!!!!!!!!!!!!!