Ok this is my understanding of it. (very simplistic)
Ok lets say that the power levers are set to ground idle, or ground fine, whatever you want to call it. And the condition levers are fully forward, which for arguments sake gives you 100% RPM. (but no forward thrust)
As you advance the power levers (increase fuel flow) the natural tendancy is for the prop RPM to increase. Now here's the clever bit. As the engine torque (twisting force produced by the gas turbine) increases, the propeller govenor adjusts the blade angle so that they are biting the air and are generating a forward thrust. This "bite" induces drag which is balanced by the increase in torque. Or to put it another way, the increased torque is transferred into thrust, rather than used to increase the prop RPM.
Pulling the power levers into reverse activates beta mode. The power levers are now controlling blade angle (reverse), while the underspeed govenor regulates fuel flow to maintain RPM.
There we go, clear as MUD
With regard to the levers,
The thrust levers control the power, or torque of the gas turbine. And the condition levers control the propeller RPM. A high RPM is normally used during takeoff, landing and reverse. A reduced RPM is used in the cruise, which also reduces noise. And for aircraft that do not have seperate propeller and condition levers, a low RPM is used during taxi.