...What the square-top sail/wingtip does is prevent spanwise flow down the low pressure side of the sail that originates from the sail top where the sail is narrow, where the leech is forward relative to the sail below. On a pin head sail or pointed wing tip, the air exiting the sail/wing on the pressure side near the tip turns 90 degrees at the trailing edge and flows or is pulled spanwise down the sail, to an area immediately below where the pressure is lower. Air wants to run downhill just like water. The air on the pressure side of the sail near the tip is at a higher pressure level than the air on the leeward side of the sail at, say, 50% chord a foot or so, down from the narrow leech above. The pressure side air at the leech sail dives vertically down the sail, leeward side into an area of low pressure on the leeward side. This spoils the delta P across the sail at the top and reduces the span effectiveness of the sail or wing. The square top moves the leech aft and this delays the opportunity for the air exiting the windward side leech to flow spanwise down the sail to a low pressure area immediately below. With this spanwise flow diminished/eliminated, the span of the sail/wing is more EFFECTIVE, it more closely approaches the ideal lift for a given span. You could say the square top sail or square wing tip is an aerodynamic dam that prevents a parasitic spanwise flow which reduces the effectiveness of a sail or wing...