If you are a fighter, you can sacrifice a shield -- and other things -- to avoid damage from a weapon or attack that does 1d6 damage. You can announce "I block with my shield!" after the successful attack roll, but you have to announce it before damage is rolled; this gives it an element of risk (maybe you'll waste your shield on a 1-point attack.) If you are attacked by an ogre or something that does 1+1 or higher damage, the shield only blocks a blow that does 6 points of damage or less; on a 7 or more, you lose the shield and take full damage. Shields do not block attacks that do 2-1 or higher (giants and such,) but there might be something that can, like a tower shield or a two-handed sword.
If you block an attack with a shield, weapon, or other object made of weaker material than the attacker's weapon, the shield is destroyed. If it's pretty much the same kind of material (bronze shield vs. iron sword,) you drop the shield and the attacker rolls a d6; on a 5+, the shield is damaged. If your shield is of stronger material, you just drop the shield, but you get to roll a d6; on 5+, your opponent's weapon is dropped (if metal or better) or shattered (if wood, bone, or weaker.)
Helmets can be sacrificed, too, and so can armor. Torso armor and armor on each limb can be sacrificed individually, but that drops your protection in that area to AC 9, if an attacker chooses to target that location specifically. Beasts don't normally target vulnerable areas, although some always target specific areas: crows go for the eyes, wolves go for the throat if they can reach it. Intelligent opponents target head, heart, or guts if these are more vulnerable, but otherwise don't target specific areas unless it's in their description.
You can sacrifice a limb instead of a shield, if you don't have one. You take an automatic result of "wounded" for that limb instead of hp loss, but the opponents rolls 1d6 anyways: on a 5+, you still take a point of damage, and the limb is "badly wounded" instead, equivalent to two wounds. Three wounds to the same limb results in permanent loss of the limb. Otherwise, "wounded" means the limb is useless for the rest of the combat and -1 (or half move) after being bandaged, "badly wounded" means useless until healed. (This is a much better, simpler version of my earlier three-wound system.)
You could, in theory, sacrifice your head or torso, but torso strikes are considered blows to the vital organs, so either case would mean death after two or three successful hits. So, you shouldn't even sacrifice your torso armor or helmet, really, unless absolutely necessary.
In theory, any body part you care to define could be handled the same way, but you can't sacrifice eyes, fingers, toes, or your nose or tongue. If an opponent targets one of these areas, it's AC 0, unless you put that body part directly into harm's way ("I walk barefoot through the weeds" or "I kiss the stone idol's mouth.")