The Kiba Dachi is also frequently called a Horse Rider’s Stance. It is often simply called a Horse Stance, a phrase which while linguistically inaccurate is still quite commonly used. The Kiba Dachi is very similar to the Shiko Dachi except that the feet are pointed toward your local octagon angle 1 and are therefore more parallel to one another when performing Kiba Dachi. This offers reasonably good stability when resisting a push or pull from the side, but offers limited resistance to a pull from the front and very poor resistance to a push from the front.
To establish this stance place your feet on the octagon three-four axis so that they are a bit more than shoulder-width apart. The exact distance will vary with your intended usage and how deeply you wish to settle into the stance. While keeping your torso erect (which is one of the more difficult aspects of this stance) bend your knees such that your torso settles directly downward.
Kiba Dachi is sometimes used when needing to resist a pushing force coming from the side. Its benefits are amplified when suddenly dropping [Settling] into a deeper stance to lower your center of gravity, and, if your attacker is bound to you in some way, pulling them out of his or her structure.
In a combative situation, the stance should almost always be considered a transitional stance. The stance (and the Shiko Dachi as well) is often held statically for long periods in practice as this helps to build leg strength and endurance that can be useful when employing this stance and in a great many other ways. You should not consider its strength-building attributes to be a tactical advantage in combat. Only use this stance as a momentary advantage and then quickly transition to something else.
Many martial arts styles practice this stance in a very deep position. The ideal position when practicing this way is often having the shins perfectly vertical to the floor and the thighs perfectly horizontal with the floor. The rear is tucked in and the head and shoulders remain back. You are encouraged to practice toward this ideal, but keep in mind two things; a) you should not lower your rear below the level of your knees to avoid undue stress on the knees, and b) this stance position has little practical value from a combative perspective since it is relatively hard to move from this stance position quickly. It looks cool, though, and will help to strengthen your legs and improve flexibility. You will not be tested on your ability to do this stance in this low manner.
If you settle deeply into the stance you will quickly begin to feel a burning in your thigh muscles. This is part of the benefit of doing this stance; it helps build excellent leg strength. But there is another way to think of this stance that is not as hard on your thigh muscles and does not result in as much fatigue. You may wish to ask your instructor about how this is done.