The tow wraps on the pivot of small diameter of green slider. It is possible that the tow passes through a hole of the pivot. One tow end is fixed to a immobile point, the other end is fixed to a point of the orange bar. Move the bar while keeping the tow strained, the center of slider pivot traces an parabola.
Basic definition: Parabola is locus of point P moving in a plane, the sum of its distances from one fixed point and from one fixed straight line is constant (the tow length).