Syncro The syncro 4 wheel drive system explained

Diffs = Differential locks the system to lock the left and right wheels together of either the front and/or rear wheels, so the drive to the wheels is sent equally to both left and right regardless of traction. Syncros in Europe usually have both front and rear wheel lock which can be independently selected from pneumatic selectors in the cab. Locks should NEVER be engaged on tarmac, only in slippery low speed situations.

VC = Viscous coupling Maintains traction to wheels with traction in the event that traction is lost on one or more other wheels. It also enables a permanent four wheel drive vehicle's wheels to rotate at different speeds when cornering. It is a viscous silicone/air and alternating plate system that allows some "slop" for normal tarmac driving.

Lloyd

Open diffs and VC mean which ever wheel on rear has least amount of traction will spin and cause the VC to engage and move power to front axle. Then which ever wheel on front axle has least amount of traction will also spin and forward momentum will stop.

Rear dif locked means both rear wheels are locked together and turn at same speed. When both rear wheels loose traction VC engages and power to front is same as rear, one wheel spins and froward momentum stops.

Front and rear locked and vc means both wheels on both axles are locked side to side and vc is only slip in system. Again rear slips causing vc to engage and front both front wheels loose traction and momentum will stop. If you leave diff locks off and you loose traction only one wheel will usually spin on each axle and you can still have control of vehicle because it's till going in a straight line, not sliding out from under you.

In icy/snowy conditions, DO NOT engage lockers because if you do when you loose traction the axle engaged will loose all control because BOTH wheels are slipping... Like trying to ice skate with no skates.

The same applies to driving on wet grass or in mud. If you are on a sidehill and have locker engaged and wheels spin, you will be sliding that axle downhill much faster then if lockers are not engaged and only one wheel spins (usually only the uphill one).

I speak from years of experience driving in severe winter conditions and off-roading with 4x4s. Limited slip diffs, power lockers, positraction, etc are double edged swords. Give you more traction to keep moving, but make it much harder to control when wheels/tires do start spinning. Especially in icy conditions. Selectable lockers like syncro have are the best choice because you can leave them in open/unlocked for slick driving conditions.

Hope that helps explain it.