Zoro struggled to fully master the technique, causing him to lose control. In order to prevent him from rampaging uncontrollably, Mihawk was forced to intervene and cut Zoro's eye. However, Mihawk did not sever Zoro's eye completely as it was infused with Haki. Instead, he struck him with enough force to subdue him. It is likely that Mihawk advised Zoro to keep his injured eye closed until he learns to control the technique or until he is in dire need of it.