You punish those who fail to press their attack.
Benefit(s): While using Upsetting Shield Style, the penalty on attack rolls foes take for being hit by your shield bash attack applies to their attacks against any target. If an adjacent foe taking this penalty makes an attack roll against any target and misses the target’s 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 as a result of this feat or Upsetting Strike each round.
Pathfinder Player Companion: Armor Master’s Handbook © 2016, Paizo Inc.; Authors: Alexander Augunas, Robert Brookes, Anthony Li, Luis Loza, and David Schwartz.