Travers, you'll also notice that the "skin as structure" - with regards to the butt and lower back - is achieved - either by arching the back and pushing the butt out, or by tucking the butt in and straightening the lower back. Those two positions have different applications, as seen in the info at the link.
Thanks Dan - like I said it was a long time ago and I'm only clarifying for my own benefit, not arguing with anyone other than myself.
(Unfortunate choice using the wiki article though - it actually states that they are different, which has added to my confusion! )
"Note that the horse stance differs from the straddle stance (四股立ち, shiko-dachi?), widely used in sumo, in which the feet point outward at 45 degrees rather than being parallel."