Easiest thing to do is google "lokka" bud :thumbs (always my first port of call for a new term)
That'll bring you up the lokka website and the 4wd Systems website - all will be revealed!
In simple terms it's an automatic differential lock which is normally on (as opposed to a manual diff lock which is normally off) and will automatically disengage when you turn a corner - though according to the experience of the guys that have them fitted (going by the lokka thread) in practice they don't unlock if the 4wd is engaged due to the power going through the diff. Not an issue though for the type of terrain you'll be on when you're in 4wd