French version
How the central difflock works
There are many systems of four wheel drive for cars:
1) Rear wheel drive with the option of engaging the two front wheels.
2) Permanent four wheel drive.
The Niva is permanent four-wheel drive, which means that there are three differentials.
One central one between the axles, one in the front axle and one in the rear axle.
Differential mechanism:
In a right hand bend, the right wheel turns slower and travels less distance than the left wheel.
The central diff works the same way as those in the front and rear axles, but between the front and rear axles instead of between the right and left wheels. The distances travelled by the front and rear axles in a bend are not the same, hence the need for a central diff.
In Practice: :
Without the central diff engaged, power is shared equally between the wheels, in theory 25% to each. The first division beween the front and the back is made by the central diff, after which each axle diff divides between left and right wheels.
In this case, if one of the four wheels is raised off the ground the car is imobilised!
or 
With the central diff engaged, the power is shared equally by the axles, 50% (in theory) to each. There is no longer any division between the front and back in the central diff, just between left and right wheels in the diffs in the axles.
In this case, if one front wheel and one rear wheel are off the ground the car is immobilised!
Options available to improve the Niva:
This kit allows the Niva to be transformed into two wheel drive.
The front axle is engaged manually to engage/disengage four wheel drive.
or
With a Promolada kit, raising one of the rear wheels off the ground is enough to immobilise the car!
This kit allows the locking of the front and/or rear diffs.
1 axle 
2 axles 
If the rear diff is locked, both the rear wheels and one of the front ones has to be off the ground to immobilise the car.
If both front and rear axle differentials are engaged, all four wheels have to be off the ground to immobilise the car!
Advantages of the Promolada kit:
Advantages of the axle differential locks :
Evident ;-)
Thanks to hoverfrog
for this translation.
