Dark Sword (暗剣殺)Compass termsNine Star Ki 九星気学

Dark Sword is by definition the direction opposite Five Yellow, and that definition is the whole of its meaning. In the logic of 気学, a force placed on one direction necessarily leaves a matching gap opposite — the harder you face one way, the less guarded your back is.

So the tradition describes it as a blade from behind. The point of that image isn't fear, it's the direction of attention: the trouble doesn't come from where you're already looking, it comes from the side you aren't.

The most useful part of this caution has nothing to do with compass points. Any time you're heavily invested in one direction, the question worth asking is what you've stopped watching. The compass simply pins that piece of common sense to a specific bearing so it can be checked.