Originally Posted by Malathar

This probably isn't exactly how it is done, but here's the logic behind it. Before each spell is cast, it runs through a number of ifchecks to determine the course of action. One of these checks is to see if the target is < 35% HP or not. If it fails, then it does the next check, if it succeeds, then it sets itself to weave soulfires.

Originally Posted by Althor

That's pretty much exactly how it works yes.
That action list is a priority list. Each entry is checked in order until it find something that is ready to cast and then it casts it and (with a few minor exceptions for some classes) then stops checking the action list until the next time it wants to cast something.