A posi-traction differential uses clutch plates that are pressed between the output spider gear and the carrier to load the spider gear. The load is applied with springs. This force is always there and is constant. This causes the wheel with grip to get power as it would be trying to overcome the friction in the clutch pack.
In a ratcheting locker (lunch box locker and most full carrier lockers), there are interlocking plates on the end of each axle which lock into a center section in the carrier. When enough force is exerted (while turning, etc), the locker will unlatch and ratchet. This will cause a noticeable tire bind/scrub while turning up to the point when it will unlatch, which typically (in my experience) is accompanied by a thunk, and then a tick as it ratchets.
The G80 diff is a compromise between an open diff and a locker, but still better than a conventional posi-trac unit. THIS VIDEO
shows how the Eaton G80 differential works quite well. While it still has the friction plates similar to a posi unit, the governor actuated cam applies force on the spider gears which press on the friction plates. When working as intended, the cam locks the spider gears to the case for full power delivery to both wheels, effectively locking them together. Also, because the G80 diff is not engaged all of the time, the it does not wear like a daily driven limited slip diff does every time you turn, even slightly.
Things get a bit muddier when you start talking about the selectable lockers that are available now. Some work on the same premise as the G80, but actuated via air, hydraulics, or electricity and are much stronger than the G80. Most act as a limited slip when unlocked and then can change to a full locker with the flip of a switch (ARB Air Locker, Eaton ECT, etc) or pull of a lever (OX). I am actually exploring using the OX lockers in my Blazer, but the e-lockers look nice too.
Not that HowStuffWorks.com is the be all and end all conversation peice, but they do have a fairly good, short description of a locker HERE
. They failed to mention that, amongst the electronic, pneumatic, and hydraulic actuation, there is also the automatic centrifugal activation that is the G80. The same premise applies.