whats possi

the tread title speaks for itself.... what is and what does it do.... i know its on the drive axle....

It locks both drive wheels so when you get on it both spin instead of just one.

posi is short for positraction it was a kind of limited-slip differential

go to howstuffworks.com and look at differentials and limited-slip differentials

here is a link-
http://auto.howstuffworks.com/differential.htm

My basic understanding of it is this:

An open differntial allows either rear wheel to turn faster than the other rear wheel while performing certain driving functions such as turning; however, with an open rear end, if one wheel looses traction, it spins and spins and spins while the one with traction hardly turns at all, resulting in the rear end fishtailing out (I believe this manuover on dry pavement is known as the "one-wheel peel").

With a closed differential, a clutch or other system limits how much one rear wheel can spin faster than the other, or locks the two wheels together which means that one cannot spin faster than the other. This means that you are less likely to fishtail with a closed differential.

Limited-slip is one form of closed differential. It uses a series of clutches to limit slip between the two rear wheels. This allows one of the rear wheels to spin a limited amount faster than the other (but not as much as an open rear end). As a result, you are less likely to fishtail with a limited-slip differential than with an open differential. This, in theory, can greatly help when driving rear wheel drive vehicles in bad weather (rain, snow, slush, etc.).