Cripple

Fallout & Fallout 2
To cripple an enemy, aim for the arms and legs. If you have a crippled leg you can't run and you use more Action Points. If you have a crippled arm you can't wield two handed weapons. If the arm crippled is the arm wielding a gun, the accuracy of the weapon will decrease significantly. If both arms are crippled, you can only use unarmed attacks.

Fallout 3
Although body part damage can be achieved through manual aiming, VATS allows more effective targeting of particular body parts. The effects of crippling specific body parts is listed below.
 * Crippling the head will reduce overall accuracy and perception of the target. If inflicted upon the player, the screen will intermittently get fuzzy and a ring will be heard through the speakers.
 * Crippling the torso will result in the target flinching far more often from shots than normal.
 * Crippling the arm will result in any weapon reliant on that arm for holding to be dropped, and will also drastically reduce accuracy. The weapon itself is not destroyed, and they will likely just pick it back up again.
 * Crippling a leg will result in impaired mobility. On creatures with a leaping attack (like deathclaws), they will no longer be able to do so.
 * Crippling the held weapon will ruin it and make it unusable, and will fly out of the target's hand. If it is an explosive ordinance (like a frag grenade), it will explode, dealing its damage to the target holding it (and any others nearby).
 * Some enemies have alternate targets: Robots, ants, and others have special targetable parts that when crippled will push them into a frenzied state, where they no longer discern between friend and foe and attack anything.

Crippling any body part will result in the target momentarily pausing to clutch the now crippled body part. Specific body parts cannot be targeted when using thrown explosives or melee/unarmed weapons in VATS.