Upsetting Strike (Combat)Source Armor Master's Handbook pg. 13
You can turn the tables on disoriented opponents.
Prerequisites: Dex 15, Combat Reflexes, Improved Shield Bash, Upsetting Shield Style, proficiency with bucklers.
Benefit: While you are using Upsetting Shield Style, if a foe is taking a penalty on attack rolls against you as a result of Upsetting Shield Style, and that foe makes a melee attack roll against you that misses your AC by 5 or more, that foe provokes an attack of opportunity from you. You can only make one attack of opportunity against an opponent in this manner each round.