Hey, This isn't really an exploit as such, but its something that you can use under many circumstances, in many situations.
Your usual Raiding warlock of present-day, is the SBSS ( Shadow Bolt Spam Spec) of 0/21/40. the "21" Points used in the demo-tree are mainly used so as to get the spell "sacrifice" enabling the warlock to sacrifice their pet, giving an increased boost to a particular stat. For a raiding warlock, they sacrifice their Sucubus so as to get the +15% shadow damage buff. When the Sucubus is initially sacrificed, the corpse doesn't despawn, allowing you to resurrect the pet causing the warlock to lose the Buff, and a Soul-Shard. Good times if you Don't like the person or just want to have fun at someone else's expense.