You can slip under a foe trying to grab you, knocking him off balance.
Prerequisites: Agile Maneuvers, Small size or smaller.
Benefit: If an opponent larger than you attempts to grapple you and fails, you may make a trip attack as an immediate action against that opponent with a +2 bonus. This does not provoke an attack of opportunity.
Advanced Player’s Guide. Copyright 2010, Paizo Publishing, LLC; Author: Jason Bulmahn.