The first time a model with a lucky charm suffers a wound in a battle they roll a D6. On a 4+ the wound is cancelled and no damage is suffered.
Superstition dictates and enforces that only a single lucky charm per the entire warband can be taken into a fight.
Lore
These take many shapes, but the most common are symbolic hammers that a pious Sigmarite Priest has touched, or carved heads of ancient Dwarf gods.