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.
Precision Shots If a model with this special rule rolls a 6 To Hit with a shooting weapon, that shot is a 'Precision Shot'.
Wounds from Precision Shots are allocated against a model (or models) of your choice in the target unit, as long as it is in range and line of sight of the firer, rather than following the normal rules for Wound allocation. A character that has a Precision Shot Wound allocated to it can still make a Look Out, Sir roll.
Note that Snap Shots and shots from weapons that scatter, or do not roll To Hit, can never be Precision Shots.