Had the crab sake at Goryu Kubo in Tokyo a few years back, which I mention in this post
They take a crab leg shell and infuse its flavors with warm/hot sake, similar to how it’s done for hire sake (fugu fin). It’s also entirely possible that there is some dashi inside, which can be used to cut into the sake in a very nice way if done right. The right profile and type of sake needs to be used (the base sake itself needs to be structured and have the ability to withstand heat, and in fact taste even better warmed) or all this won’t work as well.