User blog comment:IamJakuhoRaikoben/Chapter 403: Erza vs. Kyouka, Review./@comment-24784846-20141006235814/@comment-7418318-20141007091358

But even if you are of that opinion, that only "proofs" that Nakagami armour breaks or bends the laws of magic. It doesn't say anything about curses being affected by it or not.

If for example Demons have something other than a magic container, it could easily explain how the cuffs could work on Erza, but not on Kyouka, if the method of sealing blocking-off the container. This argument is somewhat supported by what happened between Jellal and Nadi. Jellal within the magic-sealing cell wasn't able to use magic, but the staff that Nadi held into the cell could still emit magic. Either the staff was shielded or the sealing only affects something within a mage's body, thus leaving an item or a being with a differently built body unaffected. Demons certainly have a differently build body.

Or those cuffs might even be a variation of Kyouka's own curse and prevent a mage from feeling his magic and thus the mages think they can't use magic, even though they theoretically can. Obviously Kyouka would know how to circumvent her very own curse.

Anyway, I'm not trying to say that it definitely works. I only want to point out that right now nothing is definitely prooven yet. There's just too little information.