The material below is Open Game Content from a 3PP Source
Improved Bull Rush (Combat)
You are skilled at pushing your foes around.
Prerequisite: Str 13, Power Attack, base attack bonus +1.
Benefit: You do not provoke an attack of opportunity when performing a bull rush combat maneuver. In addition, you receive a +2 bonus on checks made to bull rush a foe. You also receive a +2 bonus to your Combat Maneuver Defense whenever an opponent tries to bull rush you.
Normal: You provoke an attack of opportunity when performing a bull rush combat maneuver.
The material below is Open Game Content from the System Reference Document.
IMPROVED BULL RUSH [Combat]
Prerequisites: Str 13, Power Attack.
Benefit: When you perform a bull rush you do not provoke an attack of opportunity from the defender. You also gain a +4 bonus on the opposed Strength check you make to push back the defender.
Special: A fighter may select Improved Bull Rush as one of his fighter bonus feats.