This is my test for creating a cycloidal drive. It uses only one gear and no bearings so don't expect any wonder ;-) Has some backlash..

But it shows the concept: A gear is driven on an excentric axis which pushes the gear to the geared inside of the also geared case. Since the static teeth inside the case keep the teeth that touch the case in place which finally results in a backward rotation of the driven gear. Above that gear, there is a disk which removes that wobbling of the gear.

As a result, we got a disk which turns slow in counter-direction to the faster moving handle. In this example, the ratio is 1:19.