[jollyjumper]

differential:
when both wheels ( or axles for the center diff) are on the ground and have equal traction, the diff will devide power equally between the two output shafts. the spider gears will just push the bevel gears.
when one wheel is loose, the spider gears will will start spinning.
that loose wheel will get power, the other will stop turning.
now it comes interesting, viscous diff: there is thick oil inbetween the bevel and spider gears.
that oil has lots of resistance.
keeps the diff from slipping to some degree.
here is a toy diff ( lego)
this one has the minimum number of internal gears.