Closing Eyes
The reason Yoko's Hinman cannot fight when Yoko closes her eyes is simply that it's hard to fight when you can't see. Normal men can't fight like this either. There's no reason that Yoko closing her eyes should be mentioned here. At most, all that can be inferred from this event is that the Hinman relies upon its host's senses to control them.
P.S. Their ability to change faces is only demonstrated in the anime and is never mentioned in the novels. This is likely because it enables Sugimoto to follow Yoko around without being noticed for plot reasons. Since Sugimoto isn't in the novels, there's little reason to contrive some handwaving plot device.