Cripple

Fallout 3 and Fallout: New Vegas
Although body part damage can be achieved through manual aiming, V.A.T.S. allows more effective targeting of particular body parts. Crippling any body part will result in the target momentarily pausing to clutch the now crippled body part or reel from the injury. When using explosive and melee weapons specific body parts cannot be targeted while in V.A.T.S. The enemy is simply targeted as a whole. The effects of crippling specific body parts are listed below.
 * Crippling the head will reduce overall accuracy and Perception (-4) of the target. If inflicted upon the player character, the screen will intermittently get fuzzy and a ring will be heard through the speakers, loosely simulating the effects of a concussion.
 * Crippling the torso will result in the target flinching far more often from shots than normal. This can also affect the player character, throwing off aim and interrupting attacks (even in V.A.T.S.).
 * Weapon accuracy is reduced in V.A.T.S. by a crippled arm, but still, allow using weapons. In the first-person mode, the held weapon will sway, and while this may not seem to affect the accuracy with regular non-V.A.T.S. shots, it does make the crosshairs sway significantly while using the scopes on weapons like a sniper rifle or scoped .44 Magnum, making it much harder to aim.
 * A crippled right arm also reduces base melee damage done by 10% if wielding a one-handed weapon. If wielding a two-handed weapon (or unarmed), one crippled arm reduces base damage done by 5%, two crippled arms reduce base damage done by 10%.
 * Crippling a leg will result in impaired mobility, proportionately to being over-encumbered Creatures who rely on leaping attacks (e.g. deathclaws) will no longer be able to use them.
 * Crippling both of the player character's legs disables special melee and unarmed moves that are executed outside of V.A.T.S., such as Mauler, Khan Trick, or Ranger Takedown.
 * If both are crippled, it slows down the character further. It also gives them a limp when walking in the third person.
 * Crippling the held weapon will ruin it and make it unusable, and/or send it flying out of the target's hand. Although the condition is zero and it cannot be used, it can still be collected and repaired, or used for repairs. The player character's weapon may be damaged by enemy fire as well. If the weapon's Condition depletes, it will be unequipped as opposed to flying out of the hands. If it is an explosive ordnance (like a frag grenade), it will explode, dealing its damage to the target holding it (and any others nearby). Grenades can be targeted by V.A.T.S. while in the air after being thrown as well. If successfully hit on the fly, they will explode prematurely.
 * Some enemies have alternate targets. Robots, insects, turrets, and certain others have special targetable parts that when crippled will push them into a frenzied state where they stop discerning between friend and foe and attack indiscriminately.

Healing a crippled body part
In combat, the player character may also sustain crippling damage. In order to repair this damage, it requires:
 * Pay (50-100 caps) for a doctor to heal it.
 * Sleep in any bed.
 * Use a stimpak (either on the crippled body part directly or generally through normal stimpak-healing).
 * Develop advanced radiation sickness (400 rads) if one has the Rad Regeneration perk.
 * Use My First Infirmary.
 * Use a doctor's bag or hydra.
 * Use a healing poultice or weapon binding ritual.

Hardcore mode (Fallout: New Vegas only)

 * Paying a doctor works the same.
 * Sleeping only heals if the bed is owned.
 * Stimpaks cannot be used to heal limbs.
 * Doctor's bags function like stimpaks on the regular gameplay mode.

Fallout 4
Unlike in previous Fallout games, crippled limbs in Fallout 4 will heal on their own after a short while. In Survival mode, legs can become crippled if the Sole Survivor is over-encumbered.
 * Crippled legs prevent running and slows down the walking pace.
 * When both legs are crippled, traveling in the third person is drastically slower than traveling in the first person.

Fallout 76

 * Most enemy limbs can be crippled, preventing enemies from moving, attacking or flying. Crippling a scorchbeast's wing forces it to land. Some enemies will regenerate their crippled limbs while others such as ghouls remain crippled until killed. A legendary enemy that mutates has all of its limbs instantly restored.
 * Robots that are completely crippled will heat up and explode.
 * The Enforcer (perk) gives each shotgun pellet up to 30% chance to cripple a limb.
 * The Tormentor, Modern Renegade and One Gun Army perks also give a chance to cripple with each shot.
 * When activated, the crippling effect multiplies limb damage by 999.
 * Weapon upgrades exist that provide a 20% chance to cripple a limb.

Behind the scenes
Originally, Fallout 3 handled the crippling system by having the player cauterize their own wounds while their character scream in pain in a mini-game. The mini-game was later removed due to the system slowing down the game's pace.