On a critical threat, you can choose to forgo the bonus damage to hinder a limb for a round. Target must be declared before the critical is confirmed. If the critical fails to confirm, you do not get any bonus damage or effect.
Targeting a leg reduces an enemy’s speed by half, and targeting an arm deals a penalty to attack rolls by that arm equal to 1/2 the damage dealt. Targeting a wing forces a Fortitude save (DC = damage dealt): on a success, creature’s fly speed is reduced by half, on a failure, creature is unable to fly and, if airborne, falls toward the ground. These penalties last for one round and replace normal critical damage. Creature must be in possession of the targeted limb to receive penalty.