I think they come on when you put the car into reverse. They aren't high/low beams because both are a one-piece unit in the main Bi-Xenon lamp.

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2011 BMW X5 xDrive 35d Vermillion Red Metallic/Black Nevada Leather
Options: ZCW, ZPP, ZRC, ZTP, 300, 386, 496, 4AB, 6FL, Combox, LED Coronas. 04/11 Production