You deal extra damage if you use your shield as a weapon when charging.
Prerequisites: Improved Shield Bash, base attack bonus +3.
Benefit: If you hit an opponent with your shield as part of a charge action, in addition to dealing damage normally, you may make a trip attack without provoking an attack of opportunity. If you lose, the defender does not get to try to trip you in return.
Special: A fighter may select Shield Charge as one of his fighter bonus feats.
Source: Complete Warrior, page 105.