Ghurd built that for me a couple of years ago.
I call it the "auxiliary" diversion controller. It basically sends a "sniffer" wire to the diversion load resistor. If the main charge controller dumps through the resistor, the sense line to the Aux triggers its power circuit, which activates a relay, turning on any heavy AC load I want. The relay is held "on" for an adjustable period of time, then drops out. I use it to dump wind energy to the horses' water troughs, to keep them thawed in the winter. I live in Calgary. Minus 20 degrees. Regular 1500W 120VAC heater. It works.