A hill holder does exactly what it says on the tin. It's an automatic device that activates when you press the foot brake when the car is at a standstill usually on an incline, and once activated it will keep the car stationary even when you've removed your foot from the pedal.
Some systems hold the car for a period of a few seconds, while others will hold it indefinitely, releasing the vehicle either when you engage a manual gearbox’s first gear and start to raise the clutch pedal, or when you start to press the accelerator pedal in an automatic.
Several sensors are employed for this process to work, including angle sensors, wheel speed sensors, torque sensors and the brake actuator.
The hill holder is sometimes fitted to cars with an electronic parking brake but can also operate in tandem with a manual hand brake, depending on manufacturer and model.
It is designed to make hill starts easier by preventing rollback.
Ultimately, such a hill-hold system may be fitted to all cars in future and could replace the manual handbrake, especially if fully autonomous technology becomes a reality.
Problems with a hill holder include a refusal to release the brakes or not engaging at all, and could be caused by a mechanical issue or a problem with the car’s ECU.
Both issues should give you a warning via the dashboard and will require a visit to the dealer to fix, either covered by the car’s warranty or out of your own pocket.
Bear in mind that you will be held responsible after an accident caused by your car hitting an object as a result of a malfunctioning hill hold system, so it’s important to get a fault fixed as soon as possible.