Special Delivery is a combination game mode. For the first 30 or so seconds, you're asked to run to one end of the map and fight over a briefcase full of Australiam. The briefcase can't be picked up until after those first 30 or so seconds, and the Announcer will announce when there are only 5 seconds left.
Once the briefcase has been picked up, it becomes "owned" by a certain team. They now have to bring it over to a rocket pad. The rocket pad slowly goes up, and when it gets to the top, it blows up. I cannot tell whether or not having extra players on the pad helps or not yet.
If the person with the briefcase dies, it's dropped, and there's the usual briefcase timer. The other team cannot pick it up; it remains owned by the team that first grabbed it.
If the now defending team can keep the offensive team from picking up the briefcase until the timer expires, the briefcase resets. It returns to the part of the map it first spawned at, and it becomes neutral so either team can pick it up again.