Improved Overrun( Player's Handbook v.3.5, p. 96)
[Fighter Bonus Feat, General]
You are skilled at knocking down opponents.
Power Attack (PH) , STR 13,
When you attempt to overrun an opponent, the target may not choose to avoid you. You also gain a +4 bonus on your Strength check to knock down your opponent.
A fighter may select Improved Overrun as one of his fighter bonus feats (see page 38).
Without this feat, the target of an overrun can choose to avoid you or to block you.