I've read that the guy about to be brained wasn't the opponent, he was a bystander who offered suggestions. #justifiable Actually, it's called "Sato Tadanobu with a Goban", and the wikis say he's a samurai using a goban to defend himself when his enemies attacked. But I like the bystander offering suggestion story better.
Estampe de Yamazaki Toshinobu
TheZBlog: Playing Go (Baduk) With Korean Fishermen