:Technical Translation of Bill's Post:

the idea behind a square top is to reduce the air flowing downward (i.e. parallel with the mast and toward the water) from the top of the sail. As most of you already know, there are different pressures between the windward and the leeward side of the sail. The windward side is more normal to the regular air pressure around the boat but the leeward side actually creates a vacuum. This force literally sucks the boat forward.

On a pinhead sail, because the sail is so narrow and the leach is at such a forward angle near the top, it is easy for the higher pressure air on the windward side to curl around the sail to the low pressure side at the very top. When it does this, this higher pressure air tends to fall downward (i.e. gravity) in the vacuum on the lee of the sail and disturbs airflow and pressure for a significant portion of the top of the sail.

Enter the square top. The head of the sail is now horizontal and in line with the direction of air flow. At the highest point, the leech is not so close to the luff and the angle of the leach is more verticle. Furthermore, the point at which the air detatches from the back of the sail is also farther away from the draft where the largest difference in pressures reside. (The draft is generally the deepest part of the sail shape and is normally vertical and in the front 1/3 of the sail) . The result of all this is that the wind can detach from the back of the sail more cleanly without as much tendancy to curl around to the leeward side. The top of the sail can generate power more efficiently because the vacuum is not as disturbed.