What Antz is saying is ;

FullTime 4wd has center differential action - ie the front & rear diffs are allowed to turn at different speeds. This is becuase on road the rear of a vehicle travels less distance that the front - just like the inside wheels travel less distance than the outside wheels....make sense so far?

Ok Now that differential action means that if one end of the vehicle losses traction, all the power will be transmitted to that differential - instead of the diff with more traction.......

Then because the standard differentails are what we call "open" or "unlocked", only one wheel on the differential needs to loose traction and ALL the power that could have gone to any of the other 3 wheels is now transmitted to that one bogged wheel......
Again I hope that's clear?

NOW if we look at PART-TIME 4x4, the centre differential is "locked" so now the front & rear differentials must travel at the same speed (hence why you must NOT drive onroad in Part-time 4x4).......The locked centre diff now means that one front wheel and one rear wheel must now loose traction before you get bogged....

feel free t ask any questions if you don't understand as its pretty important stuff for a 4x4er.