This ability is best used to save a group from a wipe when aggro gets out of hand or there are adds that must be quickly and surely drawn to you. After the 6 seconds time, aggro will switch back to its original holder unless enough fresh threat was generated during that time. Good options to generate aggro on the multiple mobs attracted are: Thorns, Demoralizing Roar, and Swipe. Besides those, when grouped up, paladin Retribution Aura can be a good help.
Patch 3.0.2 (2008-10-14): Now has a 3 minute cooldown, reduced from 10 minutes. Patch 2.3.0 (2007-11-13): The chance for this ability to land successfully on its targets is now increased by hit rating.