This trick is used for solo levelling/questing mainly i suppose, seeing as the spell is a taunt, and this may work for other stuns than HoJ.
As a paladin you have Hand Of Reck. which is used to taunt mobs and deal damage.
You also have hammer of justice, when HoJ ends, the mob loses (or makes the server check) target for a moment.
This means for 0.5 secs or so, the mob will not have a target, thus you can use HoR to deal damage during this small gap.
Interestingly, it also gets misspelt (or so it seems) in the combat log when used like this — “Hand of Rekoning.”
So, in summation, Stun a mob, as the stun ends, very quickly hit HoR and the damage component of the spell will work as well as the taunt.
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World of Warcraft Cataclysm In-game Leveling Guides