The tow wraps on the pivot of small diameter of two sliders. It is possible that the tow passes through a hole of the pivot. The tow ends are fixed to rotation centers of the yellow and green bars. Turn the bars while keeping the tow strained, the center of sliders pivot traces an ellipse.
Basic definition: Ellipse is locus of point P moving in a plane, the sum of its distances from two fixed points is constant (the tow length).