Improved Trip( Player's Handbook v.3.5, p. 96)
[Fighter Bonus Feat, General]
You are trained not only in tripping opponents safely but also in following through with an attack.
Combat Expertise (PH) , INT 13,
Curling Wave Strike (Sto) , High Sword Low Axe (CW) , Improved Overrun (SF) , Knock-down (DD) , Sand Snare (Sa) , Defensive Throw (CW) , Knock-Down (SF) , Pebble Underfoot (DC) , Vae School (DrU) , Defensive Throw (OA) , Great Throw (OA) , Mighty Works Mastery II (OA) , The Gentle Way Mastery (OA) ,
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.
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 (see page 38).
Without this feat, you provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed. See Trip, page 158.