When going downhill, the driver needs to downshift to prevent the brakes from overheating.

downshift 是由前缀 down-（表示"向下"）和动词 shift（换挡）组成的复合动词，字面意思是"向下换挡"，即"降档"操作。在驾驶中特指将变速箱换到较低的档位。