On my '96 (and other years) equipped with a viscous coupler, Toyota made a system that somewhat manages itself in very basic situations. If the center differential has one DS spinning faster than the other, it heats up the VC a bit and causes a coupling to engage to balance some of the power back to the non-spinning axle. For gravel roads and snowy street driving by soccer moms this is an ideal no-think set up.
Now, if you're going to be driving without a DS as someone above mentioned - you do NOT want to drive on the VC as it is prone to heating up and binding - making it lock all the time and you unhappy with a big transfer case servicing bill. If you want to be able to drive with a DS out, you will need to get that dash CDL button in order to lock the CD in high range - there is an alternate way involving locking in low range and then pulling a fuse if you're in a pinch and don't have time to order parts.
IMHO YMMV - I am still not a mechanic.
This seems to work well in snow and slippery road conditions. It's not really intended (nor does it really work) for low-range crawling.
Cool. Makes sense. So, when driving on dry roads (read, no tire slipping) power is going 50% front and 50% rear. But once wheels start slipping, the vc tries to keep power distribution 50/50. Is that correct?
Oh, it works just fine for low range crawling. I've done plenty of stuff with the center diff open, and the VC will kick in just fine.
It's not ideal though. If you're going to be having a lot of slippage then you run the risk of damaging it. Much safer just to lock the center diff. The only times I've done it is for easy stuff where I know there will be very little slipping.
I often unlock my CD when negotiating easy terrain in low range (makes tight turns on slickrock easier), but I've never had it open under conditions of wildly spinning tires.