How many times sensei do ask to relax the shoulders during keiko or seminar. To relax the upper body. The goal of these sensei is to put across that the action of cutting is not only a physical movement of the arms but the proper usage of the entire body combined to the weight of the katana …
In Iaido it is not enough to control one enemy, if it was the case -> technique could become secondary. Iaido is being able to deal with multiple enemies, not only relying on power but on technique and adaptation. Iaido is using the body and katana in unison in order to master 4, 8, 20, 50 consecutive opponents without suki – opening – and without getting tired. “Tsune ni itte kyu ni awasu” – “Be ready at any time” even after 2 hours of intensive training.
If only the arms are used it is sure that sooner or later the physic, the muscles won’t follow anymore. The katana should not be something without consistence in your hands. The katana should be used properly, installing a dialog with it and through practice learn and understand how to use it without getting tired. You should not fight against your katana to make it do what you want, even if it weight more than 1 kg. When we start a practice session (keiko 稽古) we are full of energy and it is hard to use it as if it had no weight. A good method to understand the physical properties of a katana (weight, balance) and using it properly is to practice suburi (素振り). After some series it will quickly reveal numbers of errors linked to the over-usage of arms and shoulder power.
In Japan, Sensei use 2 terms “Wanryokou tōhō” and “Datsuryokou tōhō”. The first one alos written “Datsu-ryoku toho” designate “The sword technique using arm power” and the second one “Sword technique without strength” using inertia and gravity. Inertia and gravity are two words taking all their meaning when we think of a metal blade of 1 Kg razor sharp falling down on a target. The opponent (Teki 敵) is not made of steel but of flesh & blood. To understand the efficiency of a katana a Tameshigiri (試し斬り) session under good supervision can highlight the fact that a well-used katana does not require power.