https://books.google.com/books?id=DJR0oqh8W9YC&pg=PA42 page 42 Flaps, however, ‘pin cushion’ and develop a convex form as they contract. For this reason, flaps are best used to resurface convex surfaces. A skin graft is best for planar or concave recipient sites. … https://books.google.com/books?id=DJR0oqh8W9YC&pg=PA282 page 282 As the flap contracts, it pin cushions. This trapdoor effect can contribute to a convex shape.