It's a mental thing. Combined with his will. He might not know the exact application, but he definitely has an intention.
It's like when you punch, but you're punching with the intention to punch straight through someone face and cave in their skull, it's not going to actually happen but it will add more power when your
will is behind that image and the punch you throw.
Similarly Zoro here is creating the
intention to cut with the strength of 9 blades with the strength of a demon god, and he
imposes this visual image with his will, and therefore it is a
manifestation of his will.
Ordinarily you can't see haki, but Zoro's Asura is so strong he can literally manifest the image from his mind into his sword technique, so that others can see it:
Zoro's haki is leaking and wasting away, just wait until he learns to utilise it all and tunnel it into devastating attacks.