Safety fastening heelpiece for ski boot

A safety fastening heelpiece for a ski boot comprising a moving body mounted to pivot about an articulation axis suspended by two lateral arms linked to a base, the moving body including a release spring acting on a moving piston cooperating with a release surface of the articulation axis, so as to be able to occupy a first position corresponding to a closed position of the heelpiece in which a heel grip arranged at one end of the moving body is intended to hold the heel of the ski boot on a ski, and to be able to occupy a second position corresponding to an open position of the heelpiece, characterized in that the release surface is formed by a protrusion of the articulation axis arranged in relief relative to the rest of the articulation axis, said protrusion presenting an upper slope where the moving piston bears in the closed position of the heelpiece, and a summit forming a switchback point on which the piston remains pressed in the open position.