PFSRD:Under And Over
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Under and Over (Combat)
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.