The vehicle's secondary weapons systems carry a magazine of psychically impregnated psycannon bolts. A storm bolter or heavy bolter (twin-linked or otherwise) with this upgrade counts as AP 4 and ignores Invulnerable saves; only normal Armour saves or cover saves may be taken.
Errata — Chapter Approved: Codex: Daemonhunters Q&A (v1.0): Yes, this also ignores Invulnerable dodge saves — an Invulnerable save is an Invulnerable save, there are no subcategories.