Recovered Rage

You regain rounds of rage or raging song performance when you drop a foe.

Prerequisite: Rage or raging song† class feature.

Benefit: Whenever you reduce a foe to 0 or fewer hit points while you are raging or using raging song, you regain 1 round of rage or raging song (your choice if you have both abilities), as long as the number of Hit Dice that foe possesses is equal to or greater than 1/2  your character level. You cannot use this feat to regain more rounds of rage or raging song each day than your daily maximum number of rounds , nor can you ever exceed your maximum number of rounds.

Source: Advanced Class Guide