Do you mean the automatic main beam / dipped beam mechanism? If you use the manual main beam stalk-switch, the automatic system is cancelled for the rest of that trip. If you want to retore it during the trip, you need to pull the stalk towards you for about three seconds I think. I've found it to be very effective usually, but on the odd occasion it's a bit slow to dip, as it waits until the approaching lights are clearly visible.