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This is the focus-directrix definition of an ellipse: the set of points H equidistant from a focus and a directrix. Here the directrix is a circle and the focus is a point C inside the circle. The other focus of the ellipse is the center A of the circle. The construction also gives you the tangent line to the ellipse.
This construction accurately implements Kepler’s laws of planetary orbits. The blue vector is the velocity vector for the point moving around the ellipse. See the article by Andrew Lenard, The College mathematics journal, 1994-03, Vol.25 (2), p.90