Toyota Repair: 96 RAV 4 Center Differential Lock, rear differentials, differential lock

Question
Could you explain the purpose of the center differential lock on my 96 Rav 4.  It is a 5 speed, with all wheel drive and this lock mechanism.  When should it be used and what is its purpose?

Answer
The center differential lock will turn it into "real" 4 wheel drive, the front and rear differentials are locked to give true 4x4 drive, with the lock off the computer decides when to use 4 wheel drive which is almost never under normal driving conditions.
The center diff lock is to be used only in off-road or extreme driving conditions(mud/snow)in other words locking the differentials makes it manual four wheel drive and overrides any computer controls.