Before starting down a hill.

Slow down and shift down to a speed that you can control without using the brakes hard. Otherwise the brakes can overheat and lose their braking power.

Downshift before starting down the hill.

Make sure you are in a low enough gear, usually lower than the gear required to climb the same hill.

Continue

Special conditions where you should downshift are:

Before entering a curve.

Multi-Speed Rear Axles and Auxiliary Transmissions

Automatic Transmissions

Retarders