The Jubei story is in all likelihood fiction. IIRC, it comes from the Honcho Bugei Shoden, a compilation of stories about famous samurai that was put out in 1716, some 60 years after his death. As Shinkage-ryu was "o-ryugi", the style of the Shogun, taryu-jiai were forbidden. Yagyu Toshinaga, 20th headmaster of Yagyu Shinkage Ryu, did not believe it, as far as I know.
However, there is a phrase in Shinkage-ryu (perhaps not exclusive to it): "Abara issun" 肋一寸, which means "one inch of the ribs". The implication is that you give the opponent one inch of your ribcage, and in return you take his life.
Yet another really thought provoking quote. Give them the opening and take them when they take it.
A list of "martial wisdom" quotes like these would be great... But not being fluent enough in Japanese I have to rely on guys like you. So thanks to you all!