isRuin returns true when you break glass on a building. Tested on a few vanilla buildings and it seems to fire, not really ideal as you'd expect the building to be destroyed when this happens.
For example, I want to delete all the placed objects in a building when it's destroyed to stop them floating. Currently if you break a window, then it would clean up everything. I'd expect isRuin to only fire when the actual model changes from normal to the ruined one.