I know for sure that the PowerClean dispenses once along with the detergent release in the main wash, and again in the final rinse. I believe the same applies to the Voyager and new resource saver models.
On the PC, there are two actuator bars that are part of the dispenser assembly in the right side of the door, and they extend up to a wax motor just below the main logic board, where they are each attached separately to two small "hooks" of sorts. These two plungers operate separately, and when energized, they pull the actuator bars up. The dispenser door catch pivots and releases the door, and the rinse aid dispenser pumps a small charge of liquid out of the small port hole just inside the dispensers fill mouth.
I know that one because of restoring my PowerClean and the one my father in law now has. As for the newer style dispensers, I imagine they work in a very similar fashion, but you can look at videos such as RepairClinic to see the door disassembled.