the internals work a little soemthing like this:

you have the input coming from the gearbox, in this case, the diff via a short shaft
then you have another diff inside the casing in the picture

this diff senses the difference in rotations between the front and rear wheels

the viscous is alocked all the time, but the reason you get drive to all 4 wheels is that they are all rotating at the same time, so there is no difference in speeds of rotation between front and rear shafts (or, in this case, left and right)

if the viscous coupling fails, while you have all 4 wheels on the ground, you will have motin as the slip that the v/c is there to limit is already being limited by the fact that all 4 wheels are spinning

but as soon as you put the power down, the passenger side wheel will spin all the power away as if it were an open diff fitted vehicle OR the power will go through into the v/c, and because the v/c is fucked, the inner part won't spin the outer part so all you will be left with is the inner part spinning away like merry buggery

this can lead to the plates inside getting very hot and bangs and pops ensuing, leading to costly replacements being required

although, in the case of the pics provided, the supermarionated pictures i've got show a different set up