Complex sentence: When the gas is cooled the pressure drops and this drop means that less work needs to be done by the piston to compress the gas on the return stroke; the difference in work between the strokes yields a net positive power output.
Split into simpler sentences: When the gas is cooled the pressure drops and this drop means that the piston needs to do less work to compress the gas on the return stroke. The difference in work between the strokes yields a net positive power output.