Added with permission from Kim Taylor Nanadan (CKF) Iaido, Rokudan (CKF) Jodo and Niten Ichi Ryu Shidoin, January 2023
The path of the sword
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Until the path of the sword has been adequately studied, it is difficult to wield a sword freely at will. There is also inadequate strength.
If you do not know the spine and the surface of the sword, if it is used as a knife or a spatula for rice paste, the sword will not be attuned to the essential aim of the spirit, cutting the opponent.
In accordance with the path of the sword you should always train as if you had a heavy sword in your hand, calmly, to be able to strike your opponent well.
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For practical swordsmanship there is almost nothing as important as being able to swing the sword correctly. In the Kendo Federation grading system we find the instruction in the judging book that at 3 or 4dan the sword cut “must be correct”, but Kisshimoto hanshi at a judging seminar once told us that we don’t expect students to be able to cut until 5dan. I think I was at least 6dan when Haruna sensei spent about half an hour trying to get me to swing correctly. When I finally got it once he was delighted and I thought “what, that’s it?”. And I have now spent the last decade or more trying to remember what he showed me. To cut is not an easy thing.
The first thing to learn is the “path of the sword”, the hasuji. You can’t cut unless the edge of the sword is lined up properly. In iaido we have the grooves to make a sound when we cut. This sound can tell us where we are applying power, with what timing the power comes on, what part of the blade we are cutting with and many other things. These are all useful to an instructor and should be attended to.
Musashi talks of the spine and the edge, which is the key to getting the hasuji. The spine and the edge must be exactly on the plane of the attacking swing, if they are not, the blade becomes much wider, and at 90 degrees you may as well be holding a rice spatula, you are hitting with the flat. The small knife here is likely a reference to trying to move the tachi like a fan or a shoto as he explains in the Go Rin no Sho. Each weapon has it’s own rhythm, and if you try to move a long sword too quickly it will deviate from the path and the edge will also twist offline.
Above, Machida translates “There is also inadequate strength.” while Tokitsu says you cannot slash if you “put too much force into it”. A third translation by Haruki Kikuchi in Black Belt magazine (May 1984) states that “The swordsman who lacks power” can’t handle the sword properly. So not enough strength, too much strength… obviously strength is a concern but which is it? Be of good heart, both too much and too little is a problem. If we try to put too much strength into the sword, if we try to swing it around like a small knife, we grip too tightly and our hands become dead. The sword will not cut. If we use too little strength at the correct time the sword may also not cut, but too little strength is not usually the problem. The strongest grip and the hardest swings come at about 2dan, about two or three years into training, and for the next three or four years sensei is constantly trying to readjust grip and sensitivity to get some life back into the student’s hands.
How do we do this? How do we train to know the correct line, the correct rhythm of swing, the correct grip and shibori (the application of the grip, the pinch of the thumb and index finger) as we strike? We do as Musashi tells us, we train as if we have a heavy sword in our hands. One of my favourite sayings is that you must use a heavy sword as if it is light, and a light sword as if it is heavy. This feels like it’s very old but I got it from my two direct sensei for Iaido and Niten Ichiryu, from Ohmi Goyo and from Haruna Matsuo. Here Musashi tells us to use our sword as if it is heavy.
Feel the weight of the blade, understand the curvature, know where the tip is, let the blade adjust itself in your hand and use it according to how it wishes to be used. The lighter the sword the more delicate your grip must be in order to feel the weight. If the sword is too light or you are using too much power, you won’t have a clue where the edge is, where the tip is, or its path through the air.
You have to pick up your sword and study this well.
Seito Bugei Juku 
Awesome insights