Diff Lock workings

Ok,
How on earth does diff lock work on a defender 1995.
Does it lock the front and rear prop shafts together in the transfer box?
Do the left and right wheels still spin independantly of each other? If not how is this controlled - in the diff itself?

A newb requires a good explanation of how the system works.

thanks chaps,

p.s did the engine oil+filter, fuel + air filter, both diff oils, transfer box oil and gearbox oils yesterday..im as high as a kite on GL4 now )

Difflock is on center diff. Axles don't have lockers fitted as std. So props turn at same speed, but wheels can still spin on the axle.