Oh God, the Turboshaft debate again...
Ok, look...
The NK-12 engines on the Bear and An-22 each produce 15,000hp; ever seen that in a piston?
Anyway, the air enters the engine like a jet's, is compressed, exits the compressor inside the housing, and goes past a turbine that's connected to the propellor shaft...
To make it easier...
1)Air goes in
2)Air is compressed
3)Air exits compressor
4)Compressed air turns a turbine in the rear
5)Turbine in turn turns the shaft the propellors are connected to...
Basically a piston engine that is powered by a turbojet, rather than pistons...
A great concept, but they have to be huge for it to work correctly, and almost undoubtedly need contraprops (HUGE contraprops) to handle the power...