The description you made on your 'centre diff' is actually called a 'transfer case'. It has gears, but no reduction is made. Just changes direction of the power flow.
This description from wiki should help a little better...
http://en.wikipedia.org/wiki/Differe...hanical_device)
(I wrote out a description myself, and then somehow deleted it before posting... FML.
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---------- Post added at 07:40 AM ---------- Previous post was at 06:56 AM ----------
I've also had a look at some other threads on this site about torque distribution, and I'm sorry to say, but most of it is incorrect.
In 2 mins searching I found this...
6 MPS (MAZDASPEED6 In The U.S.)
"Active Torque Split" proactive automatic all wheel drive. Normally front-wheel drive vehicle. Electronically controlled multiplate electromagnetic clutch transfers up to 100% of torque to the rear when needed. Limited-slip differential in the rear.
The MAZDASPEED6 with advanced AWD system analyzes real-time data on steering angle, yaw rate, lateral G-force and engine status to establish up-to-minute road surface conditions, and the vehicle's dynamic status. Normal, Sport, and Snow modes give the perfect amount of torque where it is most needed, and have a performance reaction that leaves conventional gear- or fluid-controlled systems in the dust. Front/rear torque distribution varies from 100/0 to 50/50.
Source:
Mazda USA - The Official U.S. Site for Mazda Cars, Crossover Vehicles, SUVs and Trucks
The current Honda Legend, and the now extinct MDX have/had this system. (I will just quickly say now that the CR-V is a completely different setup). Simply put, it's a shaft that goes into a coupling, with wet clutch discs and metal plates. In normal operation, the input slips inside the coupling, producing NO output. When the vehicle needs it, the ECU will send a signal to the 'coupler', magnetizing the clutch plates and discs, locking them together to provide FULL output. It can be varied with lower current to allow output slip. Same output speed, just less torque.
I've just now learnt how the system works (because I haven't bothered to learn), and am trying to pass on some correctness here.
This electro-magnetic clutch system is fantastic. As I said, the Legend has this in the rear diff (instead of seperately) in the SH-AWD badged vehicles, and does the same job.
That 'switch' you have in your car to turn of the rear diff, just stops any power to the electro-magnetic clutch pack, and then enables FULL front wheel drive mode. This switch backs my description.
If you disagree at all, I'm happy to explain this better.
P.S. Just before I go, I do disagree with the extract I pulled and posted before. The electro-magnetic clutch arrangement on the 6's DOESN'T allow for 100/0% torque to the rear, it SHOULD say 100/0% torque to the front wheels, and up to 50/50 torque split, front and rear.
The 6 is primarily a FWD vehicle until rear drive is needed...