It all comes down to the Lift : Drag, or also called, Thrust:Torque Ratio.
There is a particular ratio that offers the largest amount of Thrust (push forward) to the least amount of Torque (resistance/effort in turning the propeller).
Three things determine the T:T ratio. Aircraft forward speed, RPM and Blade Angle of Attack (the size of the 'bite', as mentioned in the previous post).
By increasing the RPM past the redline, you're setting a very small bite angle and causing the engine to work extra hard (more RPM) to actually produce less thrust (forward push), which is a poor T:T ratio.
For engine management (in the real world), it's better to operate at lower RPM settings because it means less engine wear and cooler operating temps.
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