Bernoulli's theorem, also called the Longer Path Explanation, explains lift in terms of high and low air pressures on either side of the sail. Imagine the front of the boat angled upwind, or into the wind. As the breeze hits the sails, the air particles rush over both sides. Theoretically, the air particles moving across the outer, convex side of the sail have a longer distance to travel in the same amount of time as the particles moving across the inner, concave side.