Oh dear, you would have to choose the PT6 for this question wouldn't you!
I've been thinking about this question for quite a while. I'm a mechanic on PT6 powered helicopters, using various versions of the PT6. The PT6T is closest to the fixed wing engine that I work on, the other PT6 I work on is the PT6C which uses a single channel digital engine control with manual backup.
With the helicopters I work on with the PT6T engines (basically 2 PT6A engines joined together through a combining gearbox), Rotor RPM (Nf) is set by the pilot (normally at 100%) and as he pulls pitch on the blades, the engines increase power/fuel flow to try to maintain that constant RPM. Gas Generator RPM (Ng) will increase slightly to provide increased airflow through the engine so that more power can be extracted by the Nf turbine. The engine uses P3 air pressure to help regulate the actual Ng RPM.
I'll try to think fixed wing for a moment, as things are slightly different. The prop lever is used to set your RPM (Nf speed) and the power lever is used to set the power (either Ng RPM, or temperature).
With the engine in automatic mode, which is normal operation, when you decrease prop pitch/increase prop RPM, you would expect a slight decrease in load felt on the core engine (Ng), but not as much as you would expect if you decreased prop pitch but maintained prop RPM (which you would do by pushing the power lever forward).
P3 air pressure will reduce slightly, closing off the 3D cam slightly in the Automatic Fuel Control Unit (AFCU) and a slight decrease in ITT, Ng RPM, and fuel flow would be noticed. Slight reduction. Not major.
When you pull the prop levers back, what happens to your ITT/EGT/TIT? If fuel flow decreases, then temperature should decrease too, showing that the core engine doesn't have to work as hard to turn the prop.
It has been many years since I've worked on a PT6 powered aircraft, so I expect I'm pretty rusty on the nuances of how they work. Helicopter PT6's are governed to work in a totally different way, which is difficult to relate to the fixed wing engines.
PT6 engines change a lot from model to model as well. The PT6T I worked on puts out 900shp, which is getting up there. The PT6C I work on puts out over 1500shp, so you can see that there must be some big differences between these two engines! Most PT6A engines produce anywhere from 600 to 850 shp.
Perhaps a youtube search for what you are looking for would help??? Seeing as all I've probably done is confuse the issue even more