A special case of the 4-bar linkage.
Input: pink crank
Output: green crank.
The unusualness: a working cycle of the mechanism corresponds 2 revolutions of the input. Output oscillating angle is larger than 180 deg., a thing that is hard to get by using an ordinary 4-bar linkage. 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 (=10)
B: length of yellow conrod (=40)
C: length of green crank (=20)
D: distance between fixed axes of pink and green cranks (=30)
2. There are measures to overcome dead position (when green crank and yellow conrod are in line). For example, inertia of the green crank must be big enough.