Drag the black points which are the Chaikin algorithm control points. The Chaikin algorithm, named after George Chaikin, constructs the curve by generating a polygon with the corners at the given points, cutting the corners and joins the open ends so a fixed ratio of each original edge is left, and repeating the process until all corners are completely smoothed out.
I made some improvements to the controls and kupa1213 has the rest of the credit. 21/10/2020 "ghost" control points fixed.
