Kiba-dachi (literally: “middle stance”), is typically called a horseback stance, or horse stance, because it looks as though your are riding an imaginary horse (or doing the M.C. Hammer slide).
[photos of kiba-dachi from the front, and side]
A proper horse stance is much like a ready stance, except that your feet are twice as far apart (e.g., two shoulder widths). The center-of-mass must be kept extremely low in horse stance, or your stability will be severely compromised. Students will often try to escape the burning in their thighs by straightening their legs, or by leaning forward -- but they do so at their own peril. Stance quality can be easily checked by looking at your shins; your shins should make a 90° angle with respect to the floor, and the toes of both feet should point forward. Leaning forward is typically caused by sticking your butt out, as though you were twerking. This can be avoided by keeping the pelvis pushed forward as far as possible.
Just like a ready stance, one commonly enters a horse stance by “snapping” into it with a nami-ashi.
[photos of incorrect kiba dachi, from the front (high center), and side (leaning)]
Horse stance can also be used as a fighting stance. Since the side of the body contains fewer targets than the front of the body, side-facing postures are intrinsically easier to defend. However, the trade-off though is that horse stance limits the number of offensive techniques that can be used, since the rear leg and the rear hand are too far away to land techniques before the opening disappears. Even if the rear leg and hand could land in time, their ranges would be significantly compromised. When fighting out of horse stance, or any side guard position, the rear hand can do little more than act as a meat shield to cover your floating ribs.
Although fighting out of horse stance offers greater protection initially, when it fails, it fails catastrophically. It is much easier for the opponent to take control of your back in any side facing position; since it’s easier to sidestep (7-3). Fighting out of horse stance is an all-or-nothing tactic. In general, it is better to fight out of fighting stance, and keep your options open.
However, shifting to horse stance is just one of the many options fighting stance offers. For example, if you are in a fighting stance and the opponent 7-3’s outside, rather than turning, resetting in a new fighting stance, and countering; it may be faster to quickly shift into a side-facing horse stance and counter immediately.
[video explaining how to shift into side-facing from fighting stance.]
Horse stance is also a great leg exercise; have contests with friends, or against your own personal record, to see how long you can maintain this position. (Keep your hands off of your thighs; that’s cheating.)