As far as I know, all turboprop engines have mechanical continuity between the engine and the propeller except for the Pratt & Whitney Canada PT6 family of engines. What is the correct term for the setup the PT6 uses to power the propellor and how does the PT6's design differ from other turboprop engines?
Several turboprop engines, including the PT6 turboprop engine, use an epicyclic gearbox only, many other turboprop engines use a multishaft gearbox. The latter usually results in the propeller axis being offset from the turbine axis.
In common with some other engines, the PT6 turbine is also reversed. This facilitates the split-shaft arrangement where the engine and propeller are driven by separate (but in-line) shafts, each connected to their own turbine wheel.
A contrasting and common design
The difference is quite simple , with the Pratt and Whitney design there is no direct mechanical link between the turbine compressor and hot section to the propeller gearbox reduction drive . With the Allison design were you to turn the propeller by hand all the turbine parts will move all the way back to the inlet compressor . There is a direct mechanical link all the way through
As described above, it's a split shaft turboprop engine. The design isn't unique to the PT-6 (T-74) but the PT-6 variants are by far the most ubiquitous split shaft turboprops on the market today.