Yagyū Munenori was a Japanese swordsman, founder of the Edo branch of Yagyū Shinkage-ryū, which he learned from his father Yagyū "Sekishusai" Muneyoshi. This was one of two official sword styles patronized by the Tokugawa Shogunate.
Munenori began his career in the Tokugawa administration as a hatamoto, a direct retainer of the Tokugawa house, and later had his income raised to 10,000 koku, making him a minor fudai daimyo with landholdings around his ancestral village of Yagyū-zato.
Munenori entered the service of Tokugawa Ieyasu at a young age, and later was an instructor of swordsmanship to Ieyasu's son Hidetada. Still later, he became one of the primary advisors of the third shogun Iemitsu.
In about 1632, Munenori completed the Heihō kadensho, a treatise on practical Shinkage-ryū swordsmanship and how it could be applied on a macro level to life and politics. The text remains in print in Japan today, and has been translated a number of times into English.
In that masterwork under the section titled, "The Life-Giving Sword," the Master teaches us about the proper mental attitude for combat.
Returning the Mind
"The frame of mind indicated with this phrase is: if you strike with your sword and think, "I've struck!" the mind that thinks "I've struck!" will stop right there, just as it is. Because your mind does not return from the place you struck, you will be distracted, struck by the second blow of your opponent, and your initiative will be brought to nothing. With your opponent's second blow, you will be defeated.
Returning the mind means the following: if you have struck a blow, do not leave your mind in the place you struck. Rather, after you have made your strike, turn your mind back and observe your opponent. Having been struck, he will now make a strenuous effort. Having been struck, he will be mortified and insulted. If he gets angry, he will become relentless.
Here, if you are negligent, you will be struck by your opponent. It is best to think of him as an angry boar. If you think, "I struck him," your mind will stay with that thought and you will be negligent. You had best be resolved that when your opponent has been struck, he will rally. Also, once struck, your opponent will quickly become more cautious, and you will not be able to strike him with the same mind with which you struck him before. If you strike at him and miss, he will now take the initiative and strike you.
The significance of "returning your mind" is not to stop your mind where you struck, but to pull it back forcibly to your own self. It is in returning your mind and observing the countenance of your opponent. Or, in its ultimate state of mind, it is in not returning your mind at all, but in unrelentingly striking a second and third time without allowing your opponent so much as a shake of the head. This is what is called "having no space to slip in a single hair." between your first and second blow, there should be no interval that would allow in even a single hair. This is the mind with which you hit, hit, and hit again."