User blog comment:AYET/About imbued Busoshoku Haki/@comment-4889755-20121105014138

Lots of stuff has happened over 2 years. Akainu is now fleet admiral, Law is shichibukai, and the Tone dial headphones have been made.

It could be that during Zoros training, he inveted a new haki technique of putting haki power into a slash of air.

Zoro could be the inventor of this technique.