A special case of the 4-bar linkage.
Input: pink crank.
Output: green crank rotating irregularly.
The unusualness: a working cycle of the mechanism corresponds 2 revolutions of the input. It happens because:
1. The sum of the lengths of the two adjacent links is equal to the sum of the lengths of the other two links.
A + B = C + D
A: length of pink crank (=35)
B: length of yellow conrod (=15)
C: length of green crank (=40)
D: distance between fixed axes of cranks (=10)
2. There are measures to overcome dead positions (when the cranks are in line with yellow conrod). For example, inertia of the cranks must be big enough.