A character can aim to the leg of an enemy, trying to immobilize him rather than deal damage. He takes a -5 penalty to the attack roll. If the attack roll is successful and the result of the damage roll is high enough to penetrate target's DR and deal damage to its normal HP, the shot is successful. This shot doesn't deal normal damage. Instead, it deals 1 point of damage to target's HP, and the target's speed is reduced to 5 ft (if it's biped) or halved (if quadruped). Additionally, the target cannot run or charge. This condition lasts until the target is healed with DC 15 First Aid check. Biped target must make a Will save (DC 10 + the damage the shot would otherwise deal to its HP) or fall prone (at GM's discretion, if the target can lean against a wall, it shouldn't make the save). Standing up from prone while injured requires a full-round action.
Example of calculations: Enemy has 17 HP and 5 Shield HP left, and has a medium armor with DR 2. If the attack roll with -5 penalty is successful, you roll damage. If the damage is 7 or less, it's not enough to deal damage to target's HP, and the shot just deals its normal damage. For example, damage is 12. In normal attack it would deal 5 points of damage to the enemy's HP. The shot to the knee is successful. Enemy now has 17-1=16 HP, 0 Shield HP, is slowed and must make a DC 15 Will save to avoid falling prone.
I reduced the damage from this maneuver, because its main purpose is immobilization, and with normal damage it would lead to lots of "killed with a bullet in the foot" situations, especially when shooting the target that doesn't wear armor.
"That was a joke."