Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers.
A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a disaster.
A constant speed propeller in the low pitch position allows for maximum power at take off but this setting can result in a propeller overspeed at a higher airspeed, e.g. best glide speed.
Excerpt from the document:
This Special Airworthiness Information Bulletin (SAIB) alerts operators, pilots, and aircraft manufacturers of concerns for an optimum response to a propeller overspeed in piston engine aircraft with variable pitch propellers. At this time, the airworthiness concern is not an unsafe condition that would warrant airworthiness directive (AD) action under Title 14 of the Code of Federal Aviation Regulations (14 CFR) part 39.
Recently, a single-engine aircraft experienced a propeller overspeed during cruise flight at 7,000 ft altitude. The pilot reported that the application of throttle resulted in a propeller overspeed with no appreciable thrust.
To continue reading see the pdf below: