The left-side bulb is a dual-filament fixture, with the right-side bulb being a single filament. When on low beam, the dual filament bulb is burning on the lower-wattage coil. When switched to high-beam, the low coil goes out, and the high coil comes on, as well as the opposite side´s bulb. The single filament side´s reflector is designed to point down the road (spot beam). The other, designed for both flood and spot beam. I know there are conversions you can do to keep both sides lit, but a bit custom and not aftermarket. I may be wrong on the latter sentiment.