Ni-kobushi-dachi (“two-fist stance”) is the standard position for sparring. This is also the position to assume after performing a self-defense technique, in case you need to deliver a follow-up to incapacitate an especially tough opponent.
Fighting stance is the happy medium between front stance and back stance. The foot positioning is the same as either front or back stance, but with the bodyweight evenly distributed between both legs. This allows one to drive forward into a front stance (adding the power of your legs into an attack), along with the ability to shift into a back stance (to avoid blows and to force the opponent to overextend themselves).
Both knees push out to the side. This way, merely lifting a foot causes the pent-up spring tension in the other leg to automatically propel you forward and to the side, in a 7-3 movement.
The elbow of your leading arm is one fist-width away from your ribs, and the leading fist is at shoulder level, ready to throw a front-foot punch. The other hand rests in front of the navel, cocked and ready to fire off a reverse punch. Both hands are palm-up to take advantage of turning over your punches.
Both fists point inward towards your centerline (line-of-symmetry) to form a stable, triangular guard. This guard will act as a wedge, passively diverting the opponent’s attacks to either side, away from you. For this reason, it is imperative that your elbows point downward. Raising your elbows turns your strong triangle guard into a structurally-unsound pentagonal guard, while exposing your fragile floating ribs to attack. Keeping both of your hands up high like a boxer or UFC/MMA fighter isn’t good for self-defense -- those fighters train for contests in artificial microcosms where groin strikes and other low-blows are forbidden.
Do not lean forward. Leaning compromises your stability, and it makes your head easier to hit, simply because it is closer. The crouched-forward positions used in jūdō and Greco-Roman wrestling are impractical where striking is allowed. Keep the chin tucked to help protect the jaw and throat. This results in constantly looking down at angle. This is an advantage: the opponent can only be seen in the top boundary of one’s field-of-view, and peripheral vision is superior at detecting motion than central (foveal) vision; this improves one’s reaction time.
[photos of ni-kobushi-dachi from the front and side.]
When done properly, the Goshin-Jutsu fighting stance will confuse people, because it makes people look like an old-timey bare-knuckles boxer, a rare curio in today’s world. Because of this, some people won’t see this stance as a serious threat. Good -- allow your enemies to underestimate you -- the joke will be on them.