Board Thread:Naruto Discussions/@comment-26135923-20150707130017/@comment-26275777-20150709011823

The Adamantine Chains are something of a bloodline limit - Karin sort of activated them under stress. Boruto has the blood - Kushina had the chains, too - and Karin can teach him if it's actually a thing that needs to be taught. Kakashi is pretty good with seals too IIRC.

Also I imagine a lot of these things were written in scrolls and shit. Kushina left Uzushiogakure as a small child and it was destroyed not long after, but she knows quite a few of their techniques. There's an Uzumaki shrine/temple/thing on the outskirts of Konoha that holds the Shinigami masks; my headcanon is that Uzushio sent their artifacts and scrolls there for safekeeping as the war got worse for them, or that Mito brought them with her.