So the conclusion is that this combination of the shunt reference and the triac is a clever way to make a crowbar circuit. The TLV431 provides plenty of gain to make the switch point of the triac be a well defined snap point and not a soft sloppy soft turn on. This actually has some advantages as I see it over the zener diode / SCR circuit shown here by someone else which has way less gain and will end up having a soft knee turn on.