Adaptable Flanker

 
Prerequisite(s): Base Attack Bonus +4, Combat Reflexes, Vexing Flanker
 
Benefit: As a swift action, you designate a single foe as the target of this feat. When you are adjacent to the chosen target, you can choose to count as occupying any other square your threaten for purposes of determining flanking bonuses for you and your allies. You also occupy your current square for flanking an opponent.
 
Source: Players Handbook II