Propeller-by-Roger

Craftsman: Roger
Designer: Roger

One can hear something moving inside, and the propeller only moves slightly to the left and right, or “R <-> D” as seen in the picture.

I managed to solve this puzzle once now, but are having problems repeating it.

However if you wish to know what I found out, you can check the spoiler below. It will first give a description about what it is about to spoil, and then you need to click again to actually find out.

Show spoiler (it’s not the solution and never will be) »