If a heavy truck experiences sudden service brake failure on a mountain slope and has no escape ramp immediately in sight, what emergency progressive downshifting technique ('抢挡') should the driver execute?

1. Immediately double-clutch with a hard throttle blip (两脚离合中间加空油) to force the transmission into the next lower gear sequentially to bleed speed
2. Slam the gearshift directly into reverse at 70 km/h to lock the driveline
3. Turn off the ignition key, pull the steering wheel lock, and jump out of the moving cab
4. Shift the transmission into neutral and apply the trailer hand brake until the tires blow out