What happens to the power output shaft speed after it passes through the reduction gearbox?

After the power output shaft speed passes through the reduction gearbox, it reduces to a lower speed. The primary function of the reduction gearbox is to decrease the speed of the engine output to a level that is suitable for the propeller operation. This is crucial as propellers generally require a much lower speed than that produced by the engine, allowing them to function efficiently and effectively propelling the aircraft forward without exceeding their design limitations.

In terms of context, the other options do not align with how reduction gearboxes are designed to operate. For example, it wouldn't increase significantly because that would cause the propeller to rotate at an impractical and potentially damaging speed. The speed cannot stabilize at a constant speed without a variable like throttle input or flight conditions affecting it, and it certainly does not stop momentarily since continuous power transfer is essential for flight.