What (not-often-seen and not-well-known) theorem is dynamically being illustrated in the applet below? [br](Feel free to move the triangle's vertices anywhere you'd like at any time.)
For a complete description of this specific theorem involving a triangle's incircle, see this problem-set problem posted in this [url=https://twitter.com/gogeometry/status/836938468253138944]tweet from Antonio Gutierrez[/url].