Needs review
Source: Traps And Treachery 1

Description: You can use a melee weapon to make a sneak attack that inflicts subdual damage

Prerequisites: Sneak Attack

Required for: Improved Knockout Attack

Normally, a rogue cannot use a melee weapon to inflict subdual damage with a sneak attack, not even by taking the usual -4 penalty. With this feat you can use any sort of melee weapon to inflict subdual damage with a sneak attack, but the normal -4 penalty still applies.