To explain a bit of the metaphor, a piston has very discrete stages to produce work (at minimum, up and down) whereas a turbine is continuously spinning. YC's history has relied on 'stages' where all the companies are on the same path, they present at demo day, and the investors know exactly what to expect. If they turned into a 'turbine', the companies would all be at random stages, Demo Day would be much different, and YC would come to resemble more typical VC funds.