If you are leveling an alt and you get killed in pvp, instead of wait 20 looong seconds to relog on your main you can launch a second WoW (rdy with user/pwd) to connect your main char when you are attacked.
Your alt will not be instantly disconnected but if your main is near, you will maybe see your killer yet spitting on your alt. Really useful on pvp servers.
(Some of you will think that's evident but that's not, like that's not used as intended. I know that's not an exploit, but nor a leveling guide or program so i think that's the right forum section. If you don't think so, you can flame but i really think this post is more interesting than lot of others that you can find here.)