Rending claws are short and powerful, tipped with diamond-hard spikes or talons. They are quite capable of crushing plasteel and ripping through the thickest armour with their vice-like grip.
Tyranid creatures with rending claws will inflict a wound automatically and ignore armour saves, just like a power weapon, with close combat attacks which roll a 6 to hit. Other attacks that hit on a roll other than a 6 must roll to wound as normal and the victim receives their normal armour saving throw.
If a creature with rending claws rolls a 6 for its Armour Penetration dice roll against a vehicle, it rolls another D6 and adds the result to the total armour penetration score. Eg, a Tyranid Warrior hits a vehicle in close combat. The Tyranid player gets lucky and rolls a 6 for armour penetration, so he rolls the dice again and gets a 4, giving him an armour penetration score of 15 – the Warrior's Strength 5 + 6 + 4 = 15. Note that if the second roll is also a 6 the dice is not rolled again; the +12 already being added to the creature's Strength should be quite enough, thank you!
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): When rending claws are wielded by a Tyranid monstrous creature, the benefits of both stack: such a creature already ignores Armour saves in close combat, and on top of that a To Hit roll of 6 causes an automatic wound (rather than rolling to wound normally) or rolls 3D6 to penetrate a vehicle's armour. Because the target's save is ignored rather than rolled, it counts as having automatically "failed" for the purposes of triggering an Implant Attack's extra wound.