I think the HID requires the full operating voltage (12 vdc) in order for the ballast to fire the bulb. Like flourescent tubes in an office light fizture, they need their operating voltage from the ballast to fire off the tubes. If you starve the light bulb the power (by starving the ballast - only giving say, 8 volts) then the bulb cannot fire up.

I am told the DRLs on the HCH are operated by the system sending a lower voltage to the lights, which allows them to light up but not a full brightness. When you hit the brights (thats what those bulbs are used for) it send the full power required to light the lights at max.

Hopefully someone else out there can verify my response?