How does an engine rev respond when power is needed?

When power is needed, an engine revs up by increasing engine speed. This response is crucial for performance because it allows the engine to deliver more power to the drivetrain. When the throttle is opened, it allows more air and fuel into the combustion chamber, which in turn increases the engine's output. This effect is important in scenarios where additional acceleration or power is required, such as when climbing a hill, overtaking another vehicle, or towing a load.

The other responses do not align with how engines function under increased demand. Reducing speed would lead to less power, cutting off fuel would stop combustion and reduce engine power output, and cooling down is not a response to power requirements but rather a state that is important for engine operation under various conditions.