Hamstring

 
Prerequisite(s): Sneak Attack, Base Attack Bonus 4+
 
Benefit: If you hit with a sneak attack, you can choose to forego 2d6 points of bonus damage to reduce your opponents base speed by 1/2. Creatures immune to sneak attack, creatures with no legs or more than four legs are not afeected by this ability. It takes two successful hamstrings to affect a quadruped.
 
Source: Complete Warrior