Knockback( Races of Stone, p. 142)
By putting your bulk behind a blow, you can push your enemy backward.
If you score a hit while you are using the Power Attack feat, you can make a free bull rush attempt against the foe you hit, applying the number by which you reduced your attack roll as a bonus on the opposed Strength check (as well as on the damage you deal). If you hit with a twohanded weapon, you can apply double that number on the opposed Strength check. Unlike standard bull rush attempts, knockback attempts don't provoke attacks of opportunity, and you don't move with the enemy you knock backward. Bull rush rules can be found on page 154 of the Player's Handbook.
A fi ghter may select Knockback as one of his fi ghter bonus feats (see page 38 of the Player's Handbook).