"I'm afraid you're talking to the wrong Winchester for the why. I just know it works. But it only works for a short time. If they're really angry spirits they tend to find their way back to you. So you have to keep swinging." Dean scrubbed his fingers through his already mussed hair, and offered up the EMF reader. "You can have this. This will tell you whenever they're near. But it pretty much works with everything supernatural. I'll have to go to the city, but I'll get you some iron thresholds to keep them out of specific rooms, until I can figure out a way to get rid of them without remains, or giving it what it wants."