Newer landies use electrickery to brake the spinning wheel - traction control - but this is less than ideal in some situations. A true locking diff mechanism makes sure that both wheels on that axle turn at the same rate - bingo, a true three wheel drive (or with lockers both front and rear, true four wheel drive).
The ARB air locker is a locking diff that you can turn on and off while driving, using a switch to send compressed air to the diff mechanism, locking the diff. (between 80 and 105 psi pushes two plates together, overpowering the spring separating them. When the air pressure drop, the spring forces the plates apart, unlocking the diff)
The main benefit is from a locker in the rear diff. A front locker does help but get a rear locker first. The time most people install them is when they break their stock diff. A locker is stronger than a standard Landrover diff so it makes a logical replacement. The only problem is that it moves the weak link to axles and CV joints. A considerate driving style can help avoid shock loading the other drive train components - especially with a front locker.
To install one, you need to replace the diff head with the ARB unit, drill a hole in your diff casing for the air line, mount the compressor somewhere out of harms way, run the compressed air line from the diff to the compressor, fit a switch on the dash, and wire the compressor in. Sounds simple enough! In reality it took quite a while, and as I was using a second-hand locking unit (Thanks Thad!) there were a few other problems to sort out. My advice? Use Loctite that you can undo.
So, after all that, what's the difference? Some extra traction in tricky spots is great, but its not magical. You can still get stuck, just in trickier situations. In addition, lockers can have some unexpected effects on your steering. In some situations you just go straight ahead. For this reason, lockers, (especially ones in the front diff) require careful driving.
Problems - I broke the locker unit on one trip (oops!) and diff oil comes out the compressor valve every now and then. This is probably caused by pinched o-rings in the diff unit during reinstallation. I now think this may have been a false alarm - the diff oil I have traced to the diff breather, but the leak is definitely coming from the compressor motor.
June 2001 - After some carnage in the Pureoras where I broke my front diff I now have a front ARB locker as well! I have tried it out briefly and it does help in some situations - bogs and straight ascents. I'm looking forward to giving it a full testing.
June 01 - I have noticed EP90 oil coming from the compressor. The O-ring in the rear diff may be pinched, forcing oil back up the airline. The compressor has also developed an annoying slow leak. I plan to strip the compressor... sometime.
July 01 - I stripped the compressor down. It needed a full rebuild, they don't like mud too much. All is well now though! The EP90 turned out to be coming from the diff. breather which terminates close to the compressor, not the compressor itself.
January 02 - Routed the compressor on/off switch to the dash instead of the cubby box.
August 2001 - Gave the front locker a good thrashing in Thompsons - and broke a front axle (now replaced with Maxidrive heavy duty axles). Mental note - drive carefully when front locker is in! Must get some rear heavy duty axles as well.
January 2002 - Fitted warning lights on the dash that operate when the locker switches are activated - one for each locker. With the switches on the front of the cubby box, you can't see the lights on them directly. However, they are in a easily reachable position - just drop your left hand from the gearstick and there are the switches!
June 02 - Compressor died again. A new one is sitting here just waiting to be installed. Might still be able to resurrect the old one for a spare...
August 02 - Finally fitted the new one. The old one had a dud pressure switch. I will keep it and use it as a air source for pressurising the winch motor.
September 2002 - Broke the rear locker again! Replaced the whole unit, the new unit has a redesigned carrier which looks much stronger than the old design.
August 04 - Another compressor rebuild, and finally tidied up the in-cab switch.
Note: ARB has a new heavy duty airline kit that is worth getting if your are installing lockers.
If you are putting lockers in a landrover, I would only put a rear locker in a truck with 24 spline axles, and I would be very wary of putting a front locker in unless you have the large 110 size CVs and some strengthened axles. The maxidrive locker/axle/CV replacement combo althought pricey is actually worth it.