Input is the case with pulley of n1 constant velocity. Output are the pink and orange shafts of n2 and n3 velocities. The gears have the same tooth number.
The video shows when the pink shaft slows down, the orange one speeds up and vice versa to maintain the equation n1 = (n2 + n3)/2.
The parallelogram acts only when n2 or n3 differs from n1.