Ok, in basic terms, and ignoring the electronic refinement systems.....
At the end of the runway, as you push the throttles up, the increase in energy from the engine is sensed as an increase in speed by the hydro-mechanical governor system in the prop assembly. This allows hydraulic pressure to the coarse selection that moves the blades to absorb this extra speed/energy, thus maintaining a constant(ish) rpm, but delivering more power. The higher blade angle is moving more air mass, but as the rpm is constant, unlike a 'standard' jet, the measurement of power produced is torque, measured at the input to the gearbox that drives the prop.
Obviously a reduction in power setting works vice verca.
The propellor is designed to work at a set angle of attack to the oncoming airflow. As the TAS increases with altitude, the relative angle of the airflow will change, and this will also be sensed as a change in rpm by the same governor system (also corrects for IAS (and thus TAS) changes).
Accelerating down the runway, the increasing IAS (and therefore TAS) would increase the relative angle of the airflow. As the blades would therefore have less bite into the airflow, they would tend to overspeed, and once again the governor will coarsen them off to maintain the rpm.
The only time full coarse is selected is when the propellor is told to 'feather'.
Their is always a little bit of coarse pressure holding the propellor against its tendency to fine off due to the CTM/ATM battle. This tendency to want to fine off is enough to allow 'fine' or lower power settings in most cases, although full px is available to the fine line to allow the propellor to come out of the latches that hold it in feather, or to enter the ground range when the throttles are so selected.
Hope that helps.
Last edited by isaneng; 10th Mar 2011 at 13:59.
Reason: Stupidity......