Not unique to the PT6, but generic and elementary to any free-turbine engine:
The relationship between prop speed and torque is a simple one. If torque increases (assuming no change in core Ng, ITT etc) then rpm decreases. In other words, the prop governor works by controlling prop blade pitch (and thus load torque). The change in torque is what influences RPM.
The basic relationship is RPM = (horsepower / torque) X a constant
The constant depends on unique factors like the gear ratio, etc. And in a free-turbine turboprop, if the core parameters are steady, horsepower is nearly constant. Then, prop RPM is inversely proportional to torque.