Use Turbo mode by clicking on the green flag and holding down the shift key. Then you can drag your black circle with your cursor to where ever you want it to go, and then it will plug it's x and y coordinates into a fancy equation that will create a unique Julia Set.
Julia Sets were originally invented by Gaston Julia. I do not know the year. The code is a modified version based off of the "Pseudocode" from the Wikipedia article about the Mandelbrot Set. Fun Fact:, If the Mandelbrot Set is generated with multiple colors and many iterations then it will have every single Julia set hidden in different places inside of it at different zoom levels.