Yellow, violet, blue and green links create a parallelogram.
Centers of revolute joints of blue/green links creates similar triangles.
Center A of revolute joint of pink slider and center B of upper revolute joint of green link trace two lines (in red and in green) intersecting at center O of fixed revolute joint of violet link.
Angle AOB is equal to the one between two arms of blue or green link.
If A goes towards (or apart from) O, so does B.
If A moves regularly, so does B.