What must a driver do before downshifting in a manual transmission?

Before downshifting in a manual transmission, it's essential to match the engine speed with the transmission speed, often referred to as "rev matching." This process helps ensure a smooth transition between gears and prevents drivetrain shocks, which can cause wear and tear on the vehicle's components. When the engine speed is appropriately aligned with the speed of the vehicle in the lower gear, it allows for seamless engagement of the clutch and gear, maintaining stability and control.

Going through the other choices provides clarity on why they do not serve as effective steps for downshifting. Accelerating the vehicle to maximum speed is unnecessary and not conducive to the function of downshifting, as the process typically occurs when reducing speed. Ensuring the engine is at idling speed would not provide the necessary power required for the vehicle to respond effectively after shifting to a lower gear. Engaging the clutch without shifting fails to address the need for a coordinated rev matched transition, which is critical for smooth gear changes. Thus, the correct choice focuses directly on the necessary technique for effective downshifting.