Ork bioniks are typically crude but effective. Ork bionik arms are equipped with built-in weapons as standard – be they one-shot sluggas, retractable spikes, ferocious creatures in cages or something even more unsubtle. An Ork with a bionik arm causes one automatic Strength 4 hit in close combat with the equivalent of Initiative 6, but only if in base-to-base contact with the target. The plethora of spikes, blades and other hurty bits welded on to the arm mean it also counts as an additional close combat weapon. This may not be combined with mega armour.
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): A bionik arm does not count towards the "two single-handed, or one single and one double-handed" close combat weapon limit — that's the advantage of it being built in.