You know how to push opponents back.
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.
Source: Player’s Handbook (version 3.5), page 95.