There are a few different types of HID projector headlamps, but they all share the same basic premise. The light from the bulb is reflected off of a mirror and then projected through a lens. This gives the light a more focused beam, which is ideal for high-beam situations.
Some projector headlamps have bi-functionality, which means that they can be used as both high beams and low beams. This is accomplished by using a shutter to control how much light is projected. When the shutter is fully open, it allows for maximum light output and thus functions as a high beam. When the shutter is partially closed, it restricts some of the light and functions as a low beam.
One advantage of having bi-functionality in your headlamps is that it gives you more control over your light output. If you only need a low beam for most driving situations, you can keep the shutter partially closed most of the time to save on battery power. Then, when you encounter a situation where you need maximum light output (such as when driving at night in rural areas), you can open up the shutter and get full use of your headlamp's potential.
Another advantage of bi-functionality is that it allows you to use your headlamps in conjunction with other lighting sources. For example, if your car has daytime running lights (DRLs), you can wire them so that they come on automatically when the engine is started – but keep them off when the headlights are turned on. This helps to conserve battery power and prevents your DRLs from being too bright and causing glare for other drivers.