[Gokudera blinks at him in honest disbelief, then motions for Zuko to come closer. He tugs again and it tightens, then he takes his hands and tries to pull, and it tightens again. Just further illustrating that it cannot and will not come off.]

I used to work at a clinic and we helped remove pet rings from ex-pets. If it was a ring on a finger, the finger came off, and if it was an earring, then the ear came off. A bracelet, and the hand came off. Tiny, violent explosions that severed whatever the ring was attached to. I would use my flames to dumb and heal the patients there.

So... if you try to break the collar.. [he turns his hand. Welcoming Zuko to put two and two together.]