Yoroi was the armor of the samurai. The application of the techniques in the various kenjutsu kata are based on the design of the armor itself. It belongs to the fact that as a kenjutsuka you also learn about the armor itself.
In kenjutsu kata, you train with the bokken against the bokken, where you strike your bokken against the opponent’s bokken. In real combat, however, you never clash your swords, as this can destroy the blade.
All cuts and blocks in kenjutsu are really aimed at hitting the openings in the opponent’s armor. These applications for combat are called kuzushi in Katori.
For example, in the case of a cut against the kote (forearm guard), the target is the inside of the forearm (with all vessels and nerves). When cutting the neck, aim for the opening between the kabuto (helmet) and the menpo (face mask). In this opening, just below the opponent’s chin, one hits the carotid artery. The insides of the thighs are an important target (the femoral artery), as these are not protected by the haidate, the thigh protectors. Suneate (the lower leg protectors) does not protect against cuts aimed at the knee folds and calves. When hitting the torso, aim at the level of the side of the pelvis.
Many of the different postures (kamae) are also slightly different from Iaido. In battle, yoroi (armour) was worn with the accompanying kabuto (helmet). This meant that in Jodan no Kamae you had to aim the sword at a 45 degree angle above the head, otherwise the helmet would receive. For the same reason, one instead uses a lot of rolling chops, makiuchi, from the left side of the body.