Say you want to create a barrier that is unbreakable in certain circumstances. To do that, you would create highly specific and dangerous conditions that would render your barrier unbreakable. The harder it is to do, the more powerful it will be.

[ He pauses. ]

Or, if you want an ability to be particularly potent against one single person, you would create a rule that would prevent you from using it on anyone else. That sort of thing.