"Whatever may be happening with the shoulders and hips, the partners' torsos will be what Howard describes as "turned out." It is that turning out which allows the man's right and the lady's left foot to "step through." Without that turning out, the Chasse' from Promenade Position would be impossible to execute."
Ultimately, the turning out or away from the partner is limited to the feet and perhaps the hips, with the topline remaining parallel to the partner in promenade as well as 'closed' postion.
Even Alex Moore, for all his comments that have been misinterpreted to suggest turning off the partner, recognized this. For example, see his description of the chasse from promenade with its specific note that the man's body is practically square to the wall throughout (the feet are of course aligned DW). That makes the bodies, or at least the upper bodies which seem to be the specific object of this note, parallel between the partners.
The simplest practical explanation is that promenade is the compromise between the need of the partner's knees to both work in a forwards direction, and the need of their toplines to remain parallel. On the more skilled dancers, this is hardly a compromise at all, as both goals are achieved without shortfall, but in less precise settins the bodies are often seen to diverge in order to facilitate the forwardish leg/knee action when flexibility is lacking.