Well, since I basically just posted this, let me reference they way I've chosen to handle it.
-Players can make a “called shot” to a specific body part (Leg, arm, torso, or head) by self-imposing disadvantage on the attack roll. This option is not available if the player already has disadvantage from another source. A successful called shot will impose an additional effect on the target per my discretion, (Ranging from extra damage, to dropped weapons, to negative status effects such as blinded, not all creatures will be effected the same way).
It depends on the Role-playing Game system you are using as well. There may not be specific rules in every Role-playing Game, but some do have rules, and they can be adapted pretty easily from one to the next. Generally it is laid out as Txtrpg said, you take some negative modifier to hit, but get a positive modifier to damage in return. I think it is pretty fair. If you want to aim for the head, then that is one modifier, but if you want to aim for someone's eye, then that is an even greater modifier. Similarly if you want to aim for the chest, that is one thing, but if you want to aim for the heart that is even more difficult. Using this line of logic, the amount of difficultly can be established and appropriate values can be assigned.
Blinding or aiming for the head can disable the toughest of opponents rather than just doing a general attack. Weaker opponents cad do this from a distance while the front attackers / defenders make general combat moves.
The best system for this is Rune Quest because you can armor specific parts of the body and protect it in better ways than you could other parts of the body.