In this post, I will explain how to Double rocker mechanism work? As you see, it is 4 bar linkage type. What is special?
Purple link is hinged at black pin. Blue Link is hinged at red pin following link purple, hinged at pink pin following green link. Green link is hinged at yellow pin.
Blue link can make a 360° revolution, but pivoting purple and green can only oscillate, describing arcs
You can see this video below for more detail.