You are at your most dangerous when forced to protect yourself.
Benefit: When fighting defensively, you deal an extra 1d6 points of damage with any light weapon or with any weapon to which the Weapon Finesse feat applies (such as a rapier, spiked chain, or whip).
This feat’s benefit applies only when you are unarmored or wearing light armor and not using a shield.
Special: If you have the Combat Expertise feat, you also gain the benefit of Deadly Defense when taking a penalty of at least –2 on your attack roll from that feat.
A fighter can select Deadly Defense as one of his fighter bonus feats.
Source: Complete Scoundrel, page 76.