[Etzwane]

Check this article

http://people.stfx.ca/smackenz/Publi...20dynamics.pdf

Although it does not exactly address the case at hand, it shows that, in simulations, even if one does not actively supinate in the downswing the club face closes, and even can come back square if the club starts under the plane of the hands (if the other power sources are timed accordingly). I have to re-read the article to understand the reason.