Should the damper not allow engine flue gases through, the burner would automatically come on and provide heat for the water to absorb. It would do so until the controls of the damper allowed the flue gases to flow through the boiler providing the necessary heat for the water, the burner would then shut down. Rotating the propeller is the ultimate goal of any power plant. As you have probably noticed, from the text and pictures above, there is no shaft. The reciprocating steam engine.
Every action has an equal and opposite reaction.
Everything in nature reaches a balance. High pressure steam, 20bar, wants to 'go back' to being like everything else on the planet, which is water at 1 bar. If you understand the above, basic, principle, engines become very logical machines. In the earlier days the primary engine to transform the steam's heat energy to mechanical energy was done using a piston within a sealed housing. Valves in the sealed housing would allow steam to enter into the chamber, the steam restricted by the sealed housing would push on the piston, forcing it down.