Abstract: A swim fin which automatically locks into a rigid configuration when pressure is exerted against the blade during the propulsion stroke, transferring the forces and stresses past the foot to the ankle area and lower leg of the wearer, the fin having a pair of rigid brace members extending from the rear of the blade portion, with the free ends of the brace members connected to an ankle harness by a rod and slot combination and with the brace members pivotally connected to a rigid fulcrum positioned at or near the front of the foot. The slots in the brace members are offset approximately 20 to 25 degrees from the longitudinal axis of the brace members. When little or no pressure is applied to the blade, the brace members and blade can move relative to the ankle harness, but when pressure is applied to the blade, such as during the propulsion stroke, the brace members pivot on the fulcrum, automatically locking the free ends of the brace members to the ankle harness.