You are trained not only in tripping opponents safely but also in following through with an attack.
Prerequisites: 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 bonus on your Strength check to trip your opponent.
If you trip an opponent in melee combat, you immediately get a melee attack against that opponent as if you hadn’t used your attack for the trip attempt. For example, at 11th level, Tordek gets three attacks at bonuses of +11, +6, and +1. In the current round, he attempts to trip his opponent. His first attempt fails (using up his first attack). His second attempt succeeds, and he immediately makes a melee attack against his opponent with a bonus of +6. Finally, he takes his last attack at a bonus of +1.
Normal: Without this feat, you provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed. See Trip.
Special: At 6th level, a monk may select Improved Trip as a bonus feat, even if she does not have the prerequisites. A fighter may select Improved Trip as one of his fighter bonus feats.
Source: Player’s Handbook (version 3.5), page 96.