Pushback

 
Prerequisite(s): Str 17+, Improved Bullrush, Power Attack
 
Benefit: When you are adjacent to an opponent, your size or smaller, and you hit that opponent with a melee attack, you may immediately make a special bullrush check. If you succeed, you have pushed the opponent back 5' in the direction of your choice. The opponent cannot be pushed into an occupied square. This attack does not invoke an attack of opportunity, it can be used once per round.
 
Source: Miniatures Handbook