Rending If a model has the Rending special rule, or is attacking with a Melee weapon that has the Rending special rule, there is a chance that his close combat attacks will strike a critical blow. For each To Wound roll of a 6, the target automatically suffers a Wound, regardless of its Toughness. These Wounds are resolved at AP2.
Similarly, if a model makes a shooting attack with a weapon that has the Rending special rule, a To Wound roll of 6 Wounds automatically, regardless of Toughness, and is resolved at AP2.
In either case, against vehicles, each armour penetration roll of 6 allows a further D3 to be rolled, with the result added to the total. These hits are not resolved at AP2, but are instead resolved using the model/weapon's AP value.
Shred If a model has the Shred special rule, or is attacking with a Melee weapon that has the Shred special rule, it re-rolls failed To Wound rolls in close combat.
Similarly, if a model makes a shooting attack with a weapon that has the Shred special rule, it re-rolls its failed To Wound rolls.