Kokutsu Dachi is another stance primarily used in the Shotokan katas in our style.
Often described as the opposite of Zenkutsu Dachi (Forward Stance), Kokustu Dachi is effective as a defensive stance, when it's often coupled with Shuto Uke (Knife-hand Block), as in kata Bassai Dai, and in that combination it's perhaps the most classical karate-looking posture of them all.
However, the stance can also be used offensively, delivering strong blocks or angular attacks to the neck and arms. It can also be used in grappling situations as the basis for sweeps and throws.
Kokutsu Dachi offers an easy transition to Nekoashi Dachi (Cat Stance), providing fast access to a range of forward kicks.
In GKR Karate people often mistakenly call Kokustu Dachi "Back Leaning Stance," but in fact this title is a mistranslation because, whilst the centre of gravity is set back, there's no leaning involved, and certainly not backwards, which would weaken any stance considerably.
The key element of this stance is the fact that your centre of gravity should be set quite far back. Your rear leg supports approximately two-thirds (66%) of your weight, whilst the front leg supports the remainder. The back leg should be quite bent at the knee and the back foot should face outwards at a right-angle to the front. The front leg should be slightly bent and the front foot should face directly to the front. It should be two shoulder-widths forward of the rear foot, and a line drawn from the back of the rear heel would touch the inside of the front foot. Your back should be naturally straight and your body-line should be slightly forwards of your rear leg. Many stances require you to turn your hips as you strike, block or kick, but Kokutsu Dachi is unusual because it's one of few where the hips should be set at 45 degrees to the front in its relaxed position.
Students commonly get this stance wrong by lowering the body too far. This bends the rear leg further, thus forcing out the rear knee, which quickly induces pain and cramps.
Don't allow the back knee to collaps forwards
Don't lean back
Your hips should face 45 degrees not 90
Your front knee should be bent
Don't let the toes of your back foot face backwards
Keep your back elbow out at 45 degrees to the front if using a shuto guard