A steep descent can put your brakes under relentless pressure. Yet one easily overlooked letter on some automatic gear selectors can help control speed when the road drops and the corners keep coming. Before your next mountain drive, discover what that mysterious B actually does, why its behavior varies between vehicles, and how it can assist on a descent.

The “B” position on some automatic transmission selectors stands for braking or engine braking. It helps restrain the vehicle on descents, reducing reliance on the foot brake and helping limit the heat buildup that can cause brake fade. How it works depends on the vehicle: some systems use engine resistance and lower gearing, while hybrids and electric vehicles may use regenerative braking, engine braking, or a combination. The result is additional slowing when you lift off the accelerator—not a replacement for your brakes.

Before selecting B, check your owner’s manual for its intended use and any operating restrictions. It is not necessarily the best setting for everyday driving, nor does it always maximize energy recovery. Regenerative braking can also be limited when a battery is full or cold. On steep roads, choose a safe speed early, maintain space ahead, and use the brake pedal whenever necessary to stay in control.