I agree with FGD135.
Having flown a Kingair B200 I can say that the torque does increase with a reduction in Prop rpm at a constant throttle setting.
This is because there are two parts to a PT6 (as it is a free power turbine).
When reducing prop (Np) rpm the engine (N1) rpm remains the same (because it is governed to maintain the N1 rpm set by throttle position; this means that the power being produced by the engine remains constant
The torque increase can be thought of in a number of ways:
1) At reduced rpm, the same amount of exhaust gas is passing the 2nd and 3rd turbines (the Np turbines) but because of the reduced rpm, it has more time to pass over each blade and in effect imparts more force. *important note* This increased force is at a reduced propeller speed (rpm) and therefore causes no change in power (barring some negligible aerodynamic effects) but it does increase engine efficiency (which is why Long Range cruise Np is lower than Max cruise Np).
*if you want to get right into the aerodynamics, consider that each turbine blade is just an aerofoil, and as such it will have an angle of attack at which is is most efficient. At high power outputs this turbine angle of attack occurs at higher rpm (which is why you use higher rpm for climb power) and for cruise power after you have reduced power by closing the throttle somewhat, the most efficient AoA occurs at lower rpm (which is why you reduce rpm in the cruise after (or whilst) setting cruise power*
2) As stated earlier, the same amount of power is being supplied from the gas generator N1. That energy is imparted onto the Np turbines that turn the gearbox and in turn, the propeller. By reducing rpm, you have increased the prop drag and that causes more of a strain on the gear box which is displayed in the cockpit as a torque increase. (But there is no extra power)
3) *and now for the whacky scenario* Imagine pedaling your geared bike as hard as you can in 1st gear, no matter how hard you push eventually your legs will hit their rpm limit. If you push with the same effort in a higher gear your speed will increase because your leg power (e.g. engine power) is being used more efficiently.
4) In flight, with a constant throttle setting (and therefore constant N1), a decrease in rpm from climb e.g. 1900rpm in B200 and 1750rpm in B350 (I think) to the recommended cruise setting (1700rpm in B200) there is negligible change in speed but the increase in torque you spoke about
Hopefully thats as clear as mud,
TSIO540