What action is illegal when coming down a hill in a vehicle?

Coasting with the clutch in is illegal because it reduces the driver's control over the vehicle. When a driver puts the car in neutral or engages the clutch while going downhill, the engine is disconnected from the wheels, resulting in a lack of engine braking. This means that the vehicle relies solely on the brakes to control speed, which can overheat and become less effective, especially on steep inclines. Maintaining engine engagement helps the driver better manage the vehicle's speed and enhances overall control, making it a safer option in downhill driving situations.

In contrast, using your brakes intermittently allows for control while avoiding excessive brake wear, shifting gears for control provides engine braking, and maintaining a constant speed is generally safe as long as it is within legal limits.