Why and how does Taty Frans' stance, with his legs almost parallel to the water, work?


This is from Barton's video:
Apparently, Taty uses this stance even in relatively light wind. It seems to conflict with another piece of advice I remember: to keep the sail as upright as possible so that sail area is maximized.
I played around with the stance a bit during my last session, and according to the GPSLogit announcements, it was a couple of knots faster than my usual, more upright stance. The session was a bit to short for any definitive conclusions, though.
Any thoughts?