they are in a symbiotic (!) relation : the more air the propeller bites, the slower till be the RPM.
think about it in this way: if you have the engine without any propeller, and working at maximum power, the shaft gets to a certain RPM. it will NEVER went higher than that by itself (unless helped by an external force).
now, when you fit the propeller on max fine (biting the smallest amount of air), some engine force will be spent on pushing that air, so the force rotating the shaft will be smaller, therefore the RPM will go down. The more air bites, the slower it will rotate.
the exception is when you are diving, because while diving, the finest propeller setting will expose the max propeller surface against the airflow going up (as your aircraft is going down), and that will apply the biggest external force on the engine's shaft, making it exceed its maximum self obtained RPM, and breaking the engine.. that's why in dives you have to set your propeller on max coarse, to offer its smallest profile to the airflow going up, in order to not over-rev your engine.
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