In Chapter 939, Hyogoro xplained Flowing Busoshoku Haki to Luffy. Here’s what he said:
So we learned that there is an advanced type of Busoshoku Haki that is very similar to the technique Zoro taught himself during his fight against Mr 1.
This shockwave type of Advanced Busoshoku Haki is first seen when Sentomaru deflected Luffy’s attack without making direct contact.
Then we saw Rayleigh using this type of Advanced Haki. Said use of advanced Busoshoku involves channeling excess Haki from around one’s body through one’s fists. This allows the repelling of an enemy without making direct contact or using brute force.
Akainu, Kizaru and Aokiji also were seen using this advanced level of Busoshoku Haki to negate Whitebeard’s attempt to destroy the execution platform, by creating a shield combination of their three Haki.
What if the “Steel cutting technique” is related to the Advanced Busoshoku Haki showed by Akainu, Kizaru and Aokiji? What if it is simply just the invisible coat of Armament Haki applied to your sword? We all know Zoro was a strong swordsman, even before timeskip.
But it’s quite obvious Zoro got a powerup during/after Alabasta. So what I am saying is Zoro might of been using invisible Haki on his sword the entire time since Alabasta. Even he himself wouldn’t of known it was Haki at the time.
The scene where Zoro cuts Kuma with Shishi Sonson also makes sense. We all agree Kuma is pretty high tier, and for a low tier such as Zoro pre-timeskip to be able to cut Kuma, you would really need Haki.
How it is related to the deflection ability shown by Rayleigh and Sentomaru, is still to be revealed. But it’s possible the two are related.
Anyway when Hyogoro tried to explain Flowing Advanced Busoshoku Haki to Luffy, he talked about the same ability Zoro learned in Alabasta.
It’s possible that there are two types of Busoshoku Haki. It’s possible that the invisible Busoshoku Haki might be less durable, but have other benefits such as being able to use these deflection impacts. This also explains why Zoro was all of a sudden able to cut through steel.
Applying Haki to his blade, he overcame steels durability. The “cut nothing” part of his masters lesson would refer to the calmness required to use this invisible Haki.
*Theory by Visoth