Simplest way to view it is you get saved to each boss now.
So if you killed say Halfus Wyrmbreaker, then you could join any raid in The Bastion of Twilight that has also killed Halfus Wyrmbreaker. Whether they are on Valiona and Theralion, or already on Cho'gall. Whether they are 10 or 25 man. Heroic or Normal. None of that matters, the only thing that matters is if you already killed Halfus Wyrmbreaker, you can't kill him again that week. Note that if you join a raid that is further progressed than yourself though you get saved to prior bosses. So if you didn't kill Halfus Wyrmbreaker and join a raid to kill Valiona and Theralion, you would be saved Halfus Wyrmbreaker as well having participated in a raid that killed him even if you personally did not.
Once you are saved to a particular boss, you can't kill it again that week on any raid size or difficulty.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.