Vae School( Drow of the Underdark, p. 57)
You have learned House Vae's fighting style.
Once per round, when you deal damage to a flat-footed foe (or a foe you flank) with a whip or a spiked chain, you can also initiate a trip attack (as if you had hit with a touch attack). If you fail to trip when using this special attack, your opponent cannot attempt to trip you in turn.You must declare this attempt before you roll your attack, and the attempt is wasted for the round if the attack misses. You must have Weapon Focus with the weapon you are using for this feat. In other words, having Weapon Focus (whip) does not allow you to use the Vae School feat with a spiked chain, or vice versa
You must make a trip attack separately from a normal attack. If you fail to trip the foe, he can attempt to trip you in turn.