You know how to trip opponents in melee combat.

<php>echo “<b>Prerequisites:</b>”;</php> Int 13, Combat Expertise
<php>echo “<b>Benefit:</b>”;</php> You do not provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed. In addition, you receive a +4 bonus on checks made to trip a foe. You also receive a +4 bonus to your CMD whenever an opponent tries to trip you.
<php>echo “<b>Normal:</b>”;</php> A character without this feat provokes an attack of opportunity when attempting to trip a foe.