The warrior enters a berserker rage, becoming immune to Fear, Sap and Incapacitate effects and generating extra rage when taking damage. Lasts 10 sec.
Nothing incites the killing urge like the taste of one's own blood.[1]
Berserker Rage is a free self-buff that renders the warrior immune to Fear and Incapacitate effects, notably Sap, and causes extra Rage to be generated when taking damage. It lasts 10 seconds, and has a 30 second cooldown timer.
Berserker Rage is usable in all stances, and should thus be used as needed: to break fear, sap or incapacitate, or to generate extra rage from damage taken when under attack (tanking, PvP, solo).
It is worth noting that it does not actually create rage itself (unless using Improved Berserker Rage), so to get the most out of it, you will need to have aggro.
The immunities can help immensely in PvP combat, where any class who can uses Fear often. Carefully timed Fear immunity can really ruin a Warlock's day, and the Knockout immunities help defend against several Rogue abilities as well as Repentance.
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Patch 3.2.0 (2009-08-04): Tooltip clarified to indicate this ability breaks existing Fear, Sap, and Incapacitate effects. 
