Benefit

When fighting with two swords (dagger, longsword, rapier, scimitar, or short sword, in any combination), you can designate a melee opponent during your action and receive a +2 armor bonus to your Armor Class against attacks from that opponent.

This armor bonus stacks with the armor bonus from armor and shield. You can select a new melee opponent on any action. A condition that makes you lose your Dexterity bonus to Armor Class (if any) also makes you lose this bonus.