You can attack with an off-hand weapon while retaining a buckler’s shield bonus to your Armor Class.
Prerequisite: Shield Proficiency.
Benefit: When you attack with a weapon in your off hand, you may still apply your buckler’s shield bonus to your Armor Class.
Normal: Without this feat, a character wielding a buckler who attacks with an off-hand weapon loses the buckler’s shield bonus to AC until his or her next turn.
Special: A fighter may select Improved Buckler Defense as one of his fighter bonus feats.
Source: Complete Warrior, page 100.