Well, I heard a shihan who studied with o sensei say that Ueshiba Morihei refered to standing with the feet closer than usually taught today as a "stance of heaven and earth".
So my question is wether there may be an "advantage" in terms of IS.
Thank you for your detailed explanations!
I guess if "heaven and earth" in this case is implied to mean the balance of vertical forces, then it would still not be the stance that is important, per se, but the implications it had in terms of directions in forces in the body.
This is perhaps a kind of old and over-cited video, but it still is a good example of that: youtube link
. Where the boy is doing the squatting monkey exercise at the beginning, if you imagine a line dropping straight down through him representing a vertical axis of balance, then he is compressing around it, and then expanding back onto it. Then later you can see how that translates into other directions than vertical.
But with the hip joints kept in narrower width, the hip joints can more easily expand or compress about that particular line, since the legs can now extend directly under the upper body. But the benefit does not come from keeping the hips stiff and locked in that position, but rather the action of them actively balancing out the force of gravity that is acting against it - like in that upwards expanding part of squatting monkey. Like Bob was saying, it is not a bracing (= stiffened up) sort of organization you are seeking, but actively moving force between the legs and rest of the body.