When you change spec using dual specialization, the buffs that you have in one spec stay when you switch to the other spec. I can, for instance, play as SM/Ruin with the Demonic Sacrifice buff. In this case I would be able to take the Demonic Sacrifice buff, log out for my other spec, log back in and play as SM/Ruin with the extra shadow damage.
When you change spec using dual specialization, the buffs that you have in one spec stay when you switch to the other spec. I can, for instance, play as SM/Ruin with the Demonic Sacrifice buff. In this case I would be able to take the Demonic Sacrifice buff, log out for my other spec, log back in and play as SM/Ruin with the extra shadow damage.