What is 'snub braking' when descending a long, steep grade in a heavy truck?

1. Applying the trailer hand brake valve continuously to hold the speed
2. Applying the service brakes firmly to reduce speed to 5 mph below your safe speed, releasing the brakes, and repeating when speed rises back to the safe limit
3. Tapping the brake pedal rapidly to simulate an ABS system action
4. Pumping the clutch pedal to slow down the transmission output shaft