What is a critical aspect of downshifting when descending a hill?

When descending a hill, a critical aspect of downshifting is maintaining control and reducing reliance on brakes. This technique uses the engine's compression and gearing to help control the vehicle's speed and improve stability. By downshifting to a lower gear, the engine provides a braking effect known as engine braking, which helps slow the vehicle down without the excessive application of the brakes. This is essential in preventing brake overheating and potential brake failure, which can occur if the brakes are used too frequently on long or steep descents.

The practice of engine braking allows for a more controlled descent, giving the driver better handling and reaction capabilities in response to road conditions. It also extends the life of the braking system by reducing wear and tear. Therefore, effectively downshifting is not merely about changing gears; it's an integral part of safe and efficient driving when facing downhill slopes.