Okay, I don't really know if this is an exploit or just common knowledge.
This is most certainly a repost but I seriously didn't know what to search for so If it is a repost, I'm sorry. No need for flaming and shouting repost.
This require a high level character on same server depending on your alts level, preferably level 80.
Anyway, back to the subject.
Let's say you have an alt with a group quest non soloable and you have no one that can help you but you know it would be piece of cake with your main. The trick is to place your alt and your main at the group quest location, for example you are to kill a mob. I prefer to open two WoW windows, makes it a little bit easier. Make sure you are in the same party as your main. Now log in to your alt which is standing within range of the mob you're supposed to kill but be certain not to aggro. Then log your main and kill the mob as quick as you can, I believe you have 20 seconds to do so. This will give your alt credit for the kill because it takes about 20 second for it to actually disconnect from where you disconnected yourself with your main. If you know you won't make it in 20 seconds you can just repeat.
This can probably be advanced, I tried it out on bosses in UBRS too with my 80 Paladin, boosting my 60 Warrior which is on the same account and it worked perfectly with loots and everything.
Once again, I am sorry for repost and I know the text got really messy. It's because I haven't slept yet and I am really tired right now.
Sorry if the text contain any errors and such, I'm Swedish so my English isn't perfect.
I hope you get the idea!