What does a yellow triangular warning sign depicting a truck on a downward slope accompanied by the text 'Engage Low Gear' (货车使用低速挡) instruct the driver to do?

1. Shift into a low transmission gear before beginning the descent to utilize engine compression braking and retarders
2. Shift into neutral and coast down the mountain to conserve diesel fuel
3. Accelerate in the highest gear to overcome upcoming road friction quickly
4. Turn off the engine ignition to prevent engine overheating during the descent