is in its low position and the central part 26 of the shaft 25 of the drive plate 16 is positioned over the end of the bolt 18 of the pyrotechnic drive device 13 and close to the end of the bolt 18 in this same rest situation the articulated device assembly 8 is withdrawn and the first arm 11 of the device is positioned above the upper area of the central part 26 of the shaft 25 of the drive plate 16 close to the shaft 25 when a collision occurs the pyrotechnic load of the body 17 of the pyrotechnic drive device 13 is activated on the arrival of an electric signal from a sensor the bolt shaft 18 is then activated and moves from the rest position to the active position as shown in fig4 in this movement between the two positions two stages are contemplated and which occur in a continuous manner a first stage in which the bolt shaft 18 travels the distance that separates it from the central part 26 of the shaft 25 of the drive plate 16 and in its movement comes into contact with the shaft 25 driving it and moving the drive plate 16 with the movement of the drive plate 16 its bodies 24 slide all the way along the flat areas 22 of the side arms 20 of the drive guide 15 as the projections 27.