I'm not trying to derail this thread so I'll make this quick. Eventually I'll post a proper write-up on wiring HIDs. PM with questions if you have any.
A diode is an electrical check valve. It allows power flow in one direction but not the other.
I've scribbled together a little diagram to illustrate the diode method. Keep in mind, I've never actually used this method. I don't have HIDs on my cars. After studying the FSM, the diode method is the best I've come up with and the one I'll be using in the future.
As you can see, the diode is in place to allow power to come in on the high beam wire, yet does not allow power to backfeed when power is coming in on the low beam side.
This is what would happen without the diode. With the lighting switch set to low beam, power would flow to the HID but would also backfeed onto the high beam wire. Since the high beam wire is also connected to the high beam bulb, it would obviously illuminate. Without the diode, you would have no low beam setting. Both high and low bulbs would be illuminated in either switch setting.
One might argue that a diode should be installed on the low beam wire as well. The other end of the low beam wire won't be connected to anything (except the lighting switch) so the diode shouldn't be necessary.
And my thread contribution.
