Ok, I've searched the forum, blah blah, couldn't find anything. I found this on my own yesterday.
This exploit is used when you want a buff from a class which requires the buffer to be in the same group and you can't find anyone who's willing to be in the same group as you for the next hours although you have the class needed on your own account. For exeple you need a soulstone or dampen magic. I'm sure there are other uses, but the main idea is that you can buff the party with buffs that require the buffer to be in the party.
Let's say your main is a mage and you want a soulstone. No warlock is nearby and you happen to have a level 18 warlock alt on your account (but you can't be online with both to buff the other). This is how you do it:
You need help from someone else. Anyone will do the trick. Just form a party with someone. You don't have to be near each other.
You also need an addon like AtlasLoot. Other addons works as well. The idea is that you need to link something that hasn't dropped on your server yet. I use to link Warglaive of Azzinoth (drop from Illidan). An addon like lootlink that just register the links that you come accross when you play wont work. You need unsafe links.
Now, form the party. Now you're in a party with let's say a warrior. Make sure the warrior is group leader! Place your mage near the place where you have your alt warlock logged out. Link Warglaive of Azzinoth in any chat channel and you will be instantly disconnected. Quickly write your password (I have "remember account name" activated to save time) and log into your warlock alt. When you are logged in you get an invite to the party from the warrior, and you will see that your mage isn't logged out yet. Quickly buff the mage with the soulstone. Now link Warglaive of Azzinoth with your warlock and you will be disconnected again. Do not log out regularly because then your lock will leave the group. There! Now you can log back in on your mage and voilá! You are in the same party as your warlock alt and you are soulstoned! If you kick the warlock you will loose your soulstone. the warrior can be kicked though :P
This should work with other buffs that require the buffer to be in the same group, like dampen magic or amplify magic, but I haven't tried that yet. It will also work with other buffs that doesn't require the buffer to be in the same party. Like Powerword: Fortitude. If so, you can skip the part when your alt gets invited by the warrior (yes, that means that you don't need help from anyone to do it). Just buff the normal way. It also works if you want to make a portal with your mage to aid your alt. I'm sure you can figure out how to do that on your own :P
Have fun ^^