I'm not sure what you mean -- are you referring to at the end of the combustion cycle during the exhaust phase? If so, then because the exhaust channel is open,

I'm not sure what you mean -- are you referring to at the end of the combustion cycle during the exhaust phase? If so, then because the exhaust channel is open, the gases in the combustion chamber won't be exerting appreciable force on the shaft/rotor, the gasses will simply escape through the exhaust channel. At least, that's how I see it. But maybe I've understood you wrong.