Planetary speed reducer.

The orange input crank carries the green gear (Z1 = 40 teeth) that engages with stationary yellow gear (Z2 = 44 teeth). The pink output shaft has a disk of pins that engage with the holes of the green gear. The radius difference of the pink pins and the holes is equal to the eccentricity of the orange crank.

i = n1/n3 = Z1(Z2-Z1) = 10

n1: input crank velocity n3: output velocity

If (Z2-Z1) small and Z1 large, i can be very large. The input and the output rotate in opposite directions.