What (not-often-seen) geometry theorem is dynamically being illustrated in the applet below? [br][br](Feel free to move any of the white points (or outer gray point) anywhere you'd like!) [br][br]
[b]Theorem:[/b][br][br]See this [url=https://twitter.com/CutTheKnotMath/status/803271102973755392]tweet[/url] by [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url] ([url=http://www.cut-the-knot.org/]Cut-The-Knot.org[/url]).