See what?
Look at the Operating Notes for any aircraft.
If I am in cruise flight and want to achieve maximum level speed, then I must change the settings from cruise manifold pressure and rpm to a higher limited overboost manifold pressure and rpm.
Performance occurs at the specific manifold pressure and rpm setting.
You must maintain that rpm setting at a constant rate.
In a selectable pitch propeller, this is done manually by coarseing the pitch to maintain a constant rpm.
Is that hard to understand or something? It must be as we have multiple pages on this simple concept.