What is hasuji? “Hasuji” translates: “ha” – edge, “suji” – line. Technically, hasuji is the line which is drawn by the path of the edge of a sword, or, more simply, the cutting line.
The concept of proper hasuji is actually pretty simple. First, the hasuji must be straight and sharp. So this means just means good, clean lines without a yawing or a wobbling of the sword through the hasuji. And, while not the end of the world, we want to minimize the “scooping” in a cut. The concept is simple, but that does not make it easy. So what is required?
First I would say that both the left and right hands must be in line with the hasuji. For most of us – right handers – we tend to power our cuts with our right side. The imbalance between the right and left hand can cause the tip to deviate from the hasuji and spoil the cut. For this reason, I suggest focusing more on the starting and end point of the left hand. This does not mean to yank with the left, but try more to let the right hand follow the left. Slow down and let the weight of the sword do the work.
Sometimes the there are problems in kesagiri and other cuts where the sword is dropping out of the hasuji (or floating above it in other cases). What I think would be helpful is to keep you intention on your target. Don’t follow your sword around with your eyes. Also, focus on the monouchi and see it cut through the target.
Lastly, and as always, make sure that that your shoulders are in line with your hips. Breaking this harmony leads to bad hasuji, weak cuts and poor positioning.
Another concept, which is inseparable from hasuji, is edge alignment. It is said that the sword must stand up in the hasuji. Considering the cross-section of the katana, the path of the sword must follow the vector drawn from the midpoint of the mune of the sword to edge. Better put, we must align this vector with the hasuji.
Below is a cross section for a katana blade with Japanese terms for reference. The following two images illustrate good and bad edge alignment when cutting.
In the first case, the ha (edge) is in line with direction of motion (the hasuji). The forces on either side of the edge are equal so the blade passes through the medium for a successful cut. In the second case, the ha is out of alignment the force on the upper part of the edge is greater than bottom and the cut fails – the sword bounces off. Often the sword might bite the tatami, but do little else.
It is not necessary to cut tatami in order to know whether we have proper edge alignment. We can cut air. A good sign is tachikaze – “sword wind”. Tachikaze is the audible “swoosh” a sword makes as it cuts correctly though the air. The is most audible if there is a bo-hi (groove) in the sword. If not, you can still hear it if you listen closely.
What makes good or bad edge alignment? Well, beyond what was mentioned above, the most important factor here is tenouchi. “Te” – hand, “no” – of, uchi – “inside”. So, tenouchi is what is inside of the hands. A bit cryptic, but it basically means proper grip. But the grip is not static; it changes during the technique different with parts of the hand applying more or less pressure at different times.
It’s too difficult for me transcribe the details of tenouchi here so I will leave it for class for hands on instruction. Having said that, it is my opinion that tenouchi and hasuji are best learned through self-practice and experimentation.