The Prince and the Prophet

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 their close combat attacks will strike a critical blow. For each To Wound roll equal to or higher than the value listed, the target automatically suffers a Wound, regardless of its Toughness. The controlling player may choose to resolve these Wounds at AP 2 instead of the weapon’s normal AP value.
Similarly, if a model makes a Shooting Attack with a weapon that has the Rending special rule, a To Wound roll of equal to or greater than the listed value wounds automatically, regardless of Toughness, and is resolved at AP 2.
In either case, against Vehicles each Armour Penetration roll of equal to or greater than the listed value allows a further D3 to be rolled, with the result added to the total Strength of the attack. These Hits are not resolved at AP 2, but are instead resolved using the weapon’s AP value.
For example, a model with the Rending (5+) special rule that rolls To Wound against a non-Vehicle model will wound automatically on the roll of a 5+, and the attacking player has the choice of using an AP value of 2 instead of the AP value of their weapon.

A model attacking with this weapon Piles-in and fights at Initiative step 1, unless it has the Dreadnought Unit Type or Monstrous sub-type.