part 22 is on the trajectory of the lever 12 any further rotation of which it prevents despite the load exerted by the extension 10 the holding part is therefore retained in position against the force generated by the acceleration of the impact this therefore prevents any unintended opening of the door as will emerge hereinafter the movement of the mass 18 from the fig1 rest configuration to the fig2 locking configuration is effected in an irreversible manner with the result that this mass remains in the locking configuration throughout and after the impact in fig3 it is assumed that the impact phase has ended and that a user wishes to open the door from the outside the user actuates the holding part 6 applying a force having an intensity exceeding a predetermined threshold and such that the load transmitted by the lever 12 to the mass 18 forces the latter to deform the abutment 26 this movement causes the mass 18 to move from the fig2 locking configuration to the unlocking configuration shown in fig3 the user releases the holding part which causes the mass 18 to return to the rest configuration because of the effect of the spring by maneuvering the holding part 6.