You are expert in the use of shields to defend yourself.
Benefit: Whenever you fight defensively or take the full defense action and you are using a shield, you double the normal bonus to AC granted by the maneuver. Additionally, you may take the Deflect Arrows feat even if you do not meet its normal prerequisites. However, rather than having one hand free to use the Deflect Arrows feat, you must have a shield.
Adventurer’s Handbook: Genius Guide Volume 1