Now here's an idea. I have little experience of flying U.S. engines, the only ones that I flew were the JT3D, so I'm very willing to be corrected. But as I understand it some of the later American piston engines had power recovery turbines in their exhaust system which mechanically fed energy back from the gasses into the main engine. If the engine was windmilling and no fuel was being burned these turbines would become pumps sucking air out of the combustion chambers and the intake manifold as previously suggested by werbil. Comments please.