bi-xenon is hard to explain, but basically one bulb inside of a projector. The projectors have a cutoff that keeps the HID pointed lower. With bixenon when you activate your brights the projector cutoff lifts or pull back altogether releasing more HID farther down the road and not with a cutoff pattern.
I hope that made sense. one bulb low beams have cut off high beams have no cut off.
I got matching HIDs for my fogs because it looks great and allows you on a dreary day in Colorado to be seen better without running your headlights. Still have nasty aggressive looking DDE's and be seen that is.