You are able to maim a target and hinder its movement.
Benefit: Whenever you score a critical hit against an opponent, its speed is halved for 1 minute. A successful Fortitude save reduces this duration to 1d4 rounds. The DC of this save is equal to 10 + your base attack bonus. Against creatures with multiple types of movement, you must choose which movement type to affect. A flying creature hit by this attack must make a DC 10 Fly check to remain airborne, and has its maneuverability reduced by one step.
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.