The airfoil is there in all cases, in this respect; as the sail aqttacks the
wind, reguardless of the angle, some air flows on both sides of the sail.
Wind flowing over the most leeward side of the sail creates a partial vacume
on that side, so with air pressure stronger on one side the sail is
"pushed" or "drawn" to the direction where the low pressure is.
Even with the wind directly astern, a low pressure is created on the front
of the sail, and the sail(boat) is drawn forward. Hope that is as clear as
I mean it to be. Welcome to the intoxicaion of sailing.

Dave Hord.