The analogy isn’t actually entirely applicable except that the cut has to be rather precise. In both cases there needs to be a little bit left so that it’s not entirely cut in two. The difference being that in Seppuku they didn’t want to cut the head all the way through. With bagels it should be that there’s a cut deep enough that the bagel is still attached and comes apart easily. Whenever the guy who usually works the stove cuts my bagel he leaves too thick of a piece attached at the bottom, so when I try to rip them apart oftentimes a big chunk breaks off on one side instead of neatly in two. Gotta respect them knife skills.