If an item you want drops in your raid/instance but you don't win the roll and your friend or alt see's the same item drop in a separate raid/instance they can give you that item if it drops through trade.
The Items that drop are not Raid ID related but Item ID (if it dropped for you related) if you understand what I mean.
So for the people who are to stupid to understand what i mean.
1. [Item A] drops in [Raid A] and you don't win [Item A]
2. Your friend in [Raid B] wins [Item A] he can trade you [Item A]
It is kinda hard to explain hope you understand.
Enjoy
[EDIT] The two players trading have to trade within the Item Cooldown from when they downed the boss on separate raids
[EDIT] Does not work for trophies in ToC