well you need the expansion of air, but maybe there is another way using magnetism or electrostatic or something, but maybe he is just referring to how the props are mechanically turned.
You don't need to expand the air, just shove it out the back of the plane. Right now, one of the most powerful and efficient ways we have to do that is by burning fuel and causing massive thermal expansion in a nozzle that directs most of it backwards.
