"<a href=""https://www.youtube.com/watch?v=-fu03F-Iah8&amp;t=0m38s"">0:38</a> Calling it simply "a cylinder" is kinda misleading. Once you mentioned the scoops at the bottom", it made perfect sense - they're the ones doing the work after all - pulling new stuff in, which pushes everything along the screw surface. It's not some magical counter-intuitive interaction between the screw and the casing.<br>I guess, rotating the whole cylinder helps reduce the overall friction, especially on tall versions, but it should work just fine with only the scoops rotating - there is the same kind of friction in versions where the screw is rotating, and they are totally fine (although, technically, there's friction from the screw missing, so maybe combined they'd be too much, who knows). Might not even need the screw, depending on the application! :P