Improved Trip

 
Prerequisite(s): Int 13+, Combat Expertise
 
Benefit: You do not provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed. You also gain a 4 on your Strength check to trip your opponent. If you successfully trip an opponent in melee battle, you immediately get an additional attack on them ignoring their Dex bonus.
 
Source: Players Handbook