Felling Smash

Combat

You commit all your focus to a devastating blow, trying to crush your opponent to the ground.

Prerequisite: Int 13, Str 13, Combat Expertise, Improved Trip, Power Attack, base attack bonus +6.

Benefit: If you use the attack action to make a single melee attack at your highest base attack bonus while using Power Attack and you hit an opponent, you can spend a swift action to attempt a trip combat maneuver against that opponent.

Source: Ultimate Combat