Here is a quick description from the web on Spools and Minispools..

Spools & Mini Spools:
For racing, serious off-road, or crazy hot-rodders, a spool is the ultimate traction differential. Actually, it can hardly be called a differential because it does not differentiate. Spools have several purposes. First, they lock both left and right axles solid together so that there can be no loss of power due one wheel spinning faster than the other. Second, they usually support the ring gear much better than most other differentials. Last, they can be lighter than a differential, which saves rotating mass as well as un-sprung weight that the suspension has to contain.

The main shortcoming of spools is the fact that the outside wheel has to turn faster when negotiating a turn if both wheels are to maintain traction. If the axles are locked together, one or both wheels have to slip in order for the outside wheel to travel farther than the inside wheel. This can cause the rear of the vehicle to spin out, fish-tale, or can cause a lot of noise that sounds like positraction clutch chatter as the axles twist and release. Sometimes these effects are not all that bad, and they are less severe when 35-spline or larger axles are used. For those people on a budget, there are mini-spools available for many rearend designs. A mini-spool fits inside the standard open carrier, replacing the spider gears, and locking the left and right axle shafts together just like a spool does. A mini spool drives exactly like a full spool does, but they are not as strong as a full spool and do not add any extra support to the ring gear like a spool does.


Full article on description of different types of differentials can be found here.

cheers

Benny