Why should a driver partly engage the clutch, or apply the trailer brake hand valve, before releasing the service brake when starting from a stop?

Explanation

The standard technique to prevent a commercial motor vehicle from rolling backward when starting from a stop is to partly engage the clutch, or apply the trailer brake hand valve if the vehicle has one, before releasing the service brake, holding that position until enough engine power builds up to move the vehicle forward. Putting the vehicle in motion is one of the basic maneuvers CDL applicants must know under 49 CFR 383.111(a)(4)(ii), but that paragraph only names the maneuver as a required knowledge topic; it does not itself specify this clutch or hand-valve technique, which is standard driver-training practice rather than a codified federal rule. Fuel consumption and transmission warm-up are both unrelated side effects of running the engine, not the reason for this technique. Signaling other traffic that the vehicle is moving is handled by turn signals and brake lights, not by how a driver manages the clutch and brake during a start.

Source: 49 CFR 383.111(a)(4)(ii)