you have no manual control over them.the diff decides when/where to engage bit late if your stuck.atleast with arbs you have the option to engage before you enter mud/water/obstacle
Bollix!
They are wonderful!! They do not decide to "engage" they are always engaged and automatically allow the outside wheel in any corner to rotate faster than the crownwheel speed which is always the same as the inside wheel. When an outside wheel that has over-run the crownwheel i.e cornering the outside wheel is allowed to in effect ratchet around. As soon as the outside wheel slows to the same rpm as the crownwheel it locks up again i.e when driving straight. When Offroad or on road the rear wheels are ALWAYS locked.
Because of how they work it is reccommended to only fit them to the rear of a vehicle and not the front. Trying to turn a wheel while the inside wheel is driven will be hard on you arms!!
They're fool proof, bullet proof and very simple fit and forget. Nothing to wear out and no external air system to get ripped off (Make sure you get the older type if your looking for a rover one).
The downside is they do have an effect on road handling some people can live with it some cant I've had mine fitted since 2008 and its been great.
The effects they can have here:
The inside rear wheel in any corner pushes you through the bend giving sort of under steer lift off the throttle mid way and the car will tighten up in the bend. (You get used to it and its safe if you drive like a crazy person you will miss corners for sure).
Because its always pushing you around left and right hand corners from either rear wheel the rear bushes wear out quicker. Forget polybushes mine lasted 4 months. I use standard metalastic bushes which last far longer but are far less bling.
Any knackared rear suspension bushes will be very noticeable (tis a good thing tbh)
Rear tyres need to be as close as possible size circumference to each other or you will get weird handling for sure. With an open diff you can run completly different size tyres and it would be hardly noticable the diff compensates (it is its job).
In the wet if you have offroad tyres that break away easily (not too bad nowdays) you can spin up you back wheels and get the back end out very easily. Detroit lockers is what the drift boys use.
Although the above sounds really bad tis not that bad if your motor is ok to begin with.
I would say drive a motor with one fitted before buying but don't be put off.
Some people cannot bare it, me I dont go too mad hacking about so its ok was my daily driver (35 miles per day for 3 years) nowdays its a weekend / camping motor but its really not that bad.
I wouldn't fit an ARB for love nor money lots of hype and advertising for poor quality plastic internals and very small actual engagement that I've seen fail at several times at several different comps leaving the motors stranded.
I have mine in a salisbury axle far far stronger unit than a rover unit.
Have had a look at some videos and some other forums but cant seem to find exactly what I need. thanks 
