Backlash is a passive warlock ability learned at level 32 for those with the Destruction specialization. It gives the warlock a chance to gain an instant [Incinerate] after being the victim of a physical attack.
This will probably have greatest effect against melee attackers with very fast swing rate such as hunter pets and druids in cat form. Nuke to the face, close range!
Patch 3.2.2 (2009-09-22): When using Shadowbolt or Incinerate while having Backlash and Backdraft active, only Backlash will now be properly consumed.
Patch 3.1.0 (14-Apr-2009): Backlash has been moved up to tier-5, up from tier-7. Now requires Intensity (pre-requisite).
Patch 2.0.3 (09-Jan-2007): When a Warlock is under the effects of "Backlash" and "Nightfall", only one of the effects will be consumed by a "Shadow Bolt" cast.