The fastest way to sail is at a forty-five degree angle to incoming wind. I know! Ye minds have been blown! But it works. the best way to understand is to think of the wing of an airplane. It has a flat side an a rounded side. The rounded side is tough for air to get around - it has to move quicker, and there's less of it making the dash - so the air pressure is lower on that side of the wing. The flat side is easy for air to move by, and so the air pressure is higher. That pushes the plane upwards. When wind hits the sail, it puffs out, making one side rounded like an airplane's upper wing, and the other side a hollow. The air pressure in the hollow is high and in on the puff is low. The keel keeps the boat from drifting sideways, and so it moves the only way it can - forward.
So the expression "may the wind be at your back" may not be such a good thing in sailing.