Toppling Bash (Combat, Shield Mastery)Source Armor Master's Handbook pg. 19
Your shield bash throws your enemies off balance.
Prerequisites: Improved Shield Bash, Shield Focus, Stumbling Bash, base attack bonus +11 or fighter level 8th.
Benefit: As a swift action when you hit a creature with a shield bash, you can attempt a trip combat maneuver against that creature at a –5 penalty. This does not provoke an attack of opportunity, and if your attempt fails by 10 or more, you are not knocked prone.Source Armor Master's Handbook pg. 21 (Amazon)
You can spend 5 stamina points to negate the penalty on your combat maneuver check to knock a creature prone. (See page 19.)