In the right position, your shield can be used to send opponents flying.
Prerequisites
Shield Proficiency, Improved Shield Bash, base attack bonus +6.
Benefit
When you hit an opponent with a shield bash, you may initiate a bull rush attempt against them as a free action. Use your attack roll in place of the combat maneuver check. This bull rush does not provoke an attack of opportunity. Opponents who cannot move back due to a wall or other surface are knocked down after moving the maximum possible distance. You may choose to move with your target if you are able to take a 5-foot step or to spend an action to move this turn.