The Kokutsu Dachi (Back Stance) might be considered the reverse of the Zenkutsu Dachi. In the Kokutsu Dachi, the front leg is held relatively straight (it can range from significantly bent to perfectly straight, depending on the depth of the stance and its intended application) while the back leg is significantly bent. Perhaps 40% of your weight should be on the front leg while 60% of your weight is on the back leg. These percentages can vary significantly depending on the depth of your stance and your intended application.
The back foot is generally pointed in the direction of your local octagon angle 4 while the front foot points to your local angle 1. Some minor changes in these foot positions can occur as you adopt higher, lower, or wider stance positions. The toes of the two feet are generally aligned along the five-six or seven-eight octagon axis, depending on which foot is forward.
This stance might be utilized to give you a momentary positional advantage or to provide a sudden shift in your weight distribution. For example, shifting your weight back into a Kokutsu Dachi might move you back out of range of an incoming strike, but allow you to quickly return to your original position to offer a counter strike. Similarly, if you are grappling with an opponent then suddenly shifting into this stance could provide you with a weight distribution advantage that enables you to momentarily pull your opponent off balance, but then quickly shift your position to place your opponent in further structural peril.
While in this stance it is important to maintain an erect alignment of your hips, torso, and head. Being out of alignment will make your stance structurally weak and subject to failure.
The easiest way to establish this stance is to first adopt Sochin Dachi. Now shift your erect torso directly backward so that more weight is placed over your back leg.