Kōkutsu-dachi (“rear-bend stance”), or a back stance, is the complement to front stance. Unlike other karate styles, Goshin-Jutsu does not utilize a “one-line” back stance. The foot positioning is exactly the same as a front stance; only the weight distribution changes.
[photo of back stance from the front and side.]
In a back stance, 70% of the bodyweight is on the rear leg. The front leg is almost completely straight, but the front knee is not locked. Locking the knee makes you automatically slower, since it adds the extra step of unlocking the knee prior to movement. Also, locked front knee renders one vulnerable to leg sweeps.
[photo of locked knee in back stance from the front and side. with a red X, and a proper knee. from the front and side with a green checkmark.]
The rear leg is compressed, like a spring. This will feel wildly uncomfortable at first; the quadriceps will burn -- and if not, then sink lower. This tension is critical, because this is what gives karate techniques their power. Shifting from a back stance to a front stance adds of the power of your legs to all of your hand techniques. You punch with your legs -- which might sound weird -- but the video below will make this clear:
[video of punching a pad from a front stance with just arm strength, and again with proper shifting (viewed from the side).]
Shifting into a back stance also has defensive applications. By shifting from a front stance to a back stance, a normally-proportioned individual can shift their upper body a distance of one hip-width (~18”, ~45 cm) without moving their feet. An experienced karateka can use this to move to a position of safety (like a boxer’s slip), or to force a committed opponent to overextend themselves (compromising their balance).
[video (side view) of shifting back to avoid a punch. Follow with a video of a second punch, where the defender shifts back as the opponent punches, and the opponent keeps driving forward, into a crappy stance, where he gets destroyed.]
Finally, please note that the stance naming convention that Goshin-Jutsu practitioners use may differ from those of other karate styles. In Goshin-Jutsu, the term “right stance” and “left stance” indicates where the majority of the body weight resides, and not which leg is forward. So, if one were in a left front stance, then the left leg is forward; shifting one’s hips back will end in a right back stance, since the weight bears down on the right leg.