The rear end/gear/axle is called a differential because it causes the motion of the drive shaft to go in a different direction than what it started out in, i.e., it transfers the power into a 90 right angle from the original direction. A peg leg or non-posi dif will only ever spin 1 tire (if memory serves me right the left rear). A posi (positraction is a GM coined term) will turn both tires when it senses slippage from the driven tire (it uses clutches in the housing to accomplish this). A locker (a term coined from the manufacturer, Detroit Locker) stays locked until it senses a difference in tire speed, i.e., while turning corners, and then it locks, unlocks, locks, unlocks, allowing the tire on the outside to turn faster. Both posi's and locker's remain driving both tires when the car is going straight. A spool is permanently coupled to both tires. The outside tire will chirp around a corner as it's trying to go as slow as the inside tire. Hope this helped.