There is no trick to it. After you check in, you go down to the supervisor desk and ask if any upgrades are available. They will go over what they have with you and what the cost is. While it is a significant amount of savings, I don't believe anybody has every figured out if I spend this, I get this.
On our last cruise, we booked an inside stateroom on the Dream 3 night and upgraded to a concierge room for $700.