kuda in Japanese means tube.
yari in Japanese means spear.
Thus, the “tube-spear” is the simple spear, (su-yari) with the ingenious innovation of an added tube enclosing the shaft. The spear slides freely through the wooden tube, which considerably decreases friction while thrusting. To understand the implications of such an invention, imagine yourself a member of a Japanese infantry unit equipped with polearms. The time is post-Ashikaga shogunate, during the Sengoku-Jidai (the Warring States Period). By this time, the elite cavalry-based samurai had been slowly giving way to massed infantry, the new trump card of an aspiring feudal lord. Now the pikeman himself is a curious figure—he is the product of a new kind of war. Before, warfare had been the concern of a private elite, comparable to the homonoi of Sparta. However, as events proved, a peasant army with minimal training in polearm use could wear-out the brunt of a cavalry charge. It is interesting to note, however, that while the polearm could be as much as ten feet long, (the sarissa of Alexander the Great were twenty feet long,) actual thrusting distance of the yari was only about two to three feet.
Why is this?
Consider: with a polearm of such heft (ten feet long!) the hand gripping the bottom of the shaft had to stay figuratively glued to the hips and torso. Force, torque, and speed of the thrust thus depended on a dash-and-pivot strike with minimal flexing of the upper body. Should the balance of the spear stray too far from one’s center of gravity, a thrust could be easily slipped (as it often was) and a counter-attack could be successfully produced before a pikeman recovered. However, if the pikeman was clean in his thrust, there was greater accuracy and concentration of “umph” in his charge. And if the blow was slipped, a pikeman could quite feasibly pedal backwards and attempt a second strike. This however, limited even the most expert of pikeman to a thrust of three feet.
The kuda was an added bonus. Not only was friction decreased, but the distance a spear could travel through a thrust could be increased with a technique called engetsu, in Japanese meaning “Crescent Moon.” With this technique, a pikeman would penetrate a target with a twirling motion, creating a circle with the tip of his blade with about a diameter of six inches. The blade would create a devastating wound as it entered the body, and with the poor medical attention a soldier received on the battlefield, a man would most certainly die from infection. The engetsu was also useful in intercepting a thrust from an enemy and disrupting his charge. A quick thrust would then end the battle.
(Note: This node was based on information from Hunter B. Armstrong’s article, “The Sliding Yari of the Owari-Kan-Ryu.” As I’m unfamiliar with classical styles of martial arts such as the Owari-Kan-Ryu, there may be some technical errors on my part.)