This video was made based on the design numbered as 995 in volume 2, Mechanisms in modern technic, I. Artobolevski.
Angle between two sides of blue bar = angle FAO = φ
When yellow crank rotates, a side of the blue bar is always tangent to the black ellipse.
O: ellipse center.
F: a focus of the ellipse.
Semi-major axis of the ellipse = L.sin φ
L is radius of yellow crank.