The following is true for United States fighters. It is different for German fighters. At least, it was different in reality. I do not know if it is different in the simulator.
If your engine quits, lowering your R.P.M. will improve your glide slope. The lower, the better. For maximum efficiency, you do not want your R.P.M. to be much greater than your manifold pressure. A good guideline for powered flight is to keep the propeller levers and the throttles at roughly the same position if you are trying to get maximum speed for that throttle setting. If you're trying to slow down, however, raise your R.P.M.
The reason for this is that lowering your R.P.M. tells the propeller governor to maintain low R.P.M. In order to do this, the governor causes the blade angle to turn parallel to the aircraft's direction of travel, but perpendicular to the propeller's direction of rotation. This presents a broad surface to the propeller's direction of rotation, but the propeller cuts through the air in the direction of the aircraft's movement like a knife.
Raising your R.P.M. tells the propeller governor to maintain high R.P.M. It does this by changing the blade angle to be perpendicular to the aircraft's direction of travel, but parallel to the propeller's direction of rotation. This presents a broad surface to the aircraft's airflow, but the propeller cuts through the air in the direction of its rotation like a knife.
I hope I explained this clearly and properly. It took me a great deal of time to figure this out, even with much help. A diagram would, perhaps, be best, but unfortunately I cannot draw a matter of this complexity clearly enough.
An interesting fact which does not apply in simulators is that, at least under certain conditions, raising manifold pressure before raising R.P.M. and lowering R.P.M. before lowering MaP. could cause an engine failure. I believe this was especially true at high altitudes. I'm not certain what all of the conditions were, but American fighters, particuarly the P-38, had to worry about this. In addition, lowering R.P.M. in a dive was sometimes necessary to avoid over-revolving the propeller. The P-38L's maximum R.P.M. was 3200; diving at full power and R.P.M. could add several hundred R.P.M., which is bad for the engine.