Your boundless rage grants your blows extra strength, though at the expense of accuracy.
Prerequisites
Power Attack, Rage special ability.
Benefit
When you use the Power Attack feat while raging, the maximum penalty you can apply to your attack rolls while using Power Attack is increased by the number of Frenzy Feats you possess. For every point by which the penalty exceeds your base attack bonus, you deal one additional point of damage which stacks with the normal bonus from Power Attack.
For example, a 6th-level Barbarian with the Power Attack, Extra Rage, and Reckless Rage feats has a base attack bonus of +6 and 2 Frenzy Feats. He could choose to take a penalty of up to -8 on all melee attack rolls to gain up to a +10 bonus on melee damage rolls, 6 from the normal benefit of Power Attack and an additional 4 from this feat. If his chosen penalty was -6, he would gain only +6 bonus damage.
Normal
The penalty and bonus from Power Attack cannot exceed your base attack bonus.