Knocking Blows (Armor Mastery, Combat)Source Armor Master's Handbook pg. 17
The weight of your blows overwhelms your opponents.
Prerequisites: Str 13, Intense Blows, Power Attack, base attack bonus +11 or fighter level 8th, armor training class feature, proficiency with heavy armor.
Benefit: If you hit a creature that is no more than one size category larger than you with a Power Attack, the creature you attacked is also knocked off balance. Until the beginning of your next turn, it takes a –4 penalty to its CMD against combat maneuvers that move it or knock it prone.Source Armor Master's Handbook pg. 21 (Amazon)
You can spend 5 stamina points when using this feat to change the target’s penalty to CMD to –6. (See page 17.)