A lot of newer cars have 4 beam systems that already keep the low beams powered on when you engage the high beams. But there are many older cars or cars that originally came with 2 beam headlights that actually shut down the low beams when the high beams are enabled. This isn’t a big deal in halogen headlights because halogens are instant-on. HID’s require a warm-up period though, so if you turn off your high beams while driving at night, you will be driving blind while waiting for the low beams to get back to full intensity. Some ballasts like Matsushita or Mitsubishi Electric only take a couple seconds, but most aftermarket ballasts take between 7-15 seconds to warm up.
A correctional splitter is an amazing little wire harness that allows the low beams of a car to stay on with the high beams in a 4 beam system when the car doesn’t allow that functionality.
When we talk about 4 beam systems vs. 2 beam systems we mean a headlight with separate low and high beam bulbs vs. a headlight with a single bulb that has a low filament or high filament that lights up.
For example U.S. VW Jettas came with 9007 2 beam headlights. Just one bulb in each housing for the low and high beams. In Europe there was the option of a HID housing that had a dedicated low-beam projector and a high beam halogen reflector. When you install the European lights on a U.S. car, it doesn’t know that the headlight has a dedicated low and high beam, so it will shut down the low beams when you turn on the high beams.