Gaastra designed the Inset Clew to take advantage of a shorter boom without compromising the sails aerodynamics. As sails generate lift, high pressure air from the windward side of the sail tends to move towards the lower pressure air on the leeward side as it clears the trailing edge. Because of this, air at the head of the sail tends to go from the windward side to the leeward side over the top of the sail creating a tip vortex. This vortex creates drag that slows the sail down.
Competing brands achieve inward clew position by cutting away the leech and foot of the sail beneath the boom creating another wingtip vortex. Gaastra have designed the Inset Clew without cutting away any more than is necessary to fit the boom. The wind exits the trailing edge cleanly and no additional tip vortex forms.