Theres several kanji variations such as: 心妙剣. And this term is found in most koryu in different section, and different descriptions.
The current theory's are that:
1) shinmyōken is the term that refers to winning without fighting. Not being in the wrong place at the wrong time, solving problems with diplomacy, etc.
2) a particular fighting technique, as presented in Yagyū Shinkage-Ryū, where the enemy is knocked down and their will to fight taken without taking their life.
3) a sense of timing such as shuhari: before, during, and after.
It is my suspicion that it is a blend of the three.
I am also running a poll on this on our Facebook page: https://www.facebook.com/BCMARA
Here is the entry from Yagyu Shinkage-ryu heiho e-mokuroku for Shinmyoken 神妙剣
Dr. Kacem Zoughari recently explained in one of his Sunday classes the following:
"
Here in the case of what we practice, if we look carefully, the primary scrolls deal with that, particularly the Shinkage-ryū, the Kage mokuroku, one of the main words that was used was Shinmyōken. Shin (kami; 神), the Japanese word… kami some people translate as divine, but it’s a little bit different. Myō can be the light (明) or it can be the subtlety (妙), like bimyō (subtlety; 微妙). Finally, Ken (剣), it’s the Japanese double-edged sword, not katana (刀). So, when you say Shinmyōken, it’s like this subtle sword, this subtle double-edged sword from the kami, or if you used the kanji of light, it’s the light of the kami (神明), or the lightning sword of the kami (神明剣). But if you translate like this literally, it means nothing honestly speaking. For applying something that is for killing people. You must think that this word was created in order to show something more profound, deeper; this is the aspect of Shinmyōken – the centerline.
So, the centerline is first how to be centered, to understand your own center, and understanding your own center will allow you to understand the center of your opponent. It comes from the use of the weapon, because if you don’t keep you center with the line, if you’re too open, then you’re going to get wounded or killed on the battlefield. Being on one line allows you to protect yourself and minimize all the kyūsho and weak points. You can even learn this in shooting as well, whether with a rifle or pistol; line in duels back in the day, they would stand profile because they would try to minimize the surface. Nowadays people stand broadly to the target, but that’s nowadays, I’m explaining something that goes way back. "