First of all please note that this damage is based on your health. The more health you have the more damage it will do. This little trick only works because of the combined mechanics of Touch of Karma - Spell - World of Warcraft and Censer of Eternal Agony - Item - World of Warcraft.
How it Works
As you may or may not know, the censer mentioned above almost instantly inflicts 90% of your health in damage to yourself after its cast time.
There's a small interval between the actual cast of the buff and said damage. ToK simply needs to be used in this interval. It is super easy because you can just mash your bind for ToK as the cast is about to finish. Not super crazy or anything but it's pretty funny watching an enemy player's health tick away. Of course this can only be done every 10 minutes as u need the censer.
Pics:
imgur: the simple image sharer
imgur: the simple image sharer