I know the questions been put out there before so i figured i'd post a link to some really good explinations. I happened to be playing on Mitusbishi's commercial sites and found this...does a nice job of explaining plus it gives a visual right down to the mV readings.
http://www.meaa-mea.com/products/p_vvt.asp
http://www.meaa-mea.com/products/p_vvt.asp
