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It is called a winglet, and it is used to reduce vortex drag. Vortex drag occurs at the end of the wings. Underneath the wings you have higher pressure air that is trying to force its way upwards. At the end of the wing, the air succeeds. The wing’s forward motion spins this upward spill of air into a long spiral, like a small tornado, that trails off the wing tip, creating vortex drag.