Improved Overrun( Sword and Fist: A Guidebook to Monks and Fighters, p. 7)
You are trained in knocking over opponents that are smaller than you.
When you attempt to overrun an opponent who is at least one size category smaller than you, the target cannot avoid you. If you knock down your opponent, you immediately get an attack of opportunity against that opponent, gaining the standard +4 bonus on attack rolls against prone targets.
The target of your overrun attack chooses to avoid you or block you.