Excuse the spelling....but one must understand Bernoullis Principle. That states as the velocity of a fluid (air) increases, it's pressure decrease.

Think of runners running around a track, save for a staggerded starting line, the outside runners run a further distance correct? If they start from a straight line, and finish in a tie, then it IS safe to say the outside runner ran FASTER than the inside (more distance, same amount of time)....

Think of our sail now...as the air seperates over the sail, it travels "faster" over the lee (curved) section of the sail than on the inside.

Now apply the principle, as the velocity increases, the pressure decreases, which means we have a low pressure on the outside and a higer (when compared to the outside) pressure on the inside. Mother nature does not like imbalance so the high tries to go to the low, the sail is in the way...

Force to move our boat!!!

We can even accelerate that intital velocity by adding a jib (leading edge slot) to make an even greater pressure differential...

Hope this helps...