The 'Pos Shift' line entry is just a position updating function. If you are (for example) using a displaced threshold for take-off, or an intersection rather than full length, the distance from the normal threshold is entered in hundreds of metres on the 'Pos Shift' line. Having done so, when the TOGA switch is pressed the FMS will position update to that revised position rather than the usually defined runway threshold. Gives a more accurate starting point for subsequent navigation.