Using this enables you to use multiple abilities from the same Skills page at the same time. IE 2 defensive abilities at once or 2 Rage abilities at once.
So I am sure a good amount of people here have figured out how to do this on their own and have been doing it regularly. I however just seen my buddy pop Wrath of the beserker, call of the ancients, and earthquake at the same time and was like.. WTF lol. I do not know the usefullness of this to other classes as I have only played a barb so far. But as a barb this makes life ALOT easier during big fights. I am not even certain that this is working as intended?
Go to Game Menu > Options > Gameplay > Elective Mode (The bottom check box on the right side).
Now simply right click the hotkey at the bottom of your screen you want to link to a particular ability. You don't need to use all 3 ultimates, you can also set up for multiple shouts or to have cleave and frenzy up at the same time.
After right clicking the hotkey you want to link to the window will show up. If you clicked the hotkey for defensive abilities, the defensive abilities window will come up. Using the arrows to the right and left of the abilities will pan you to the other groups of abilities, select the ability you want and are now able to set up all of your ultimate abilities to be used on seperate cool downs.
Hopefully this helps some folks out, I know it has made my life alot easier =).