Psychic Orks

Although it may not be immediately apparent, all Orks are psychic - their minds constantly bubbling with psychic energy. This innate psychic activity protects them from daemons and other manifestations of the warp. [...] Orks in battle are always chanting, stamping their feet and brandishing their weapons in an enthusiastic fashion. The anticipation and excitement soon gets their psychic minds bubbling away, so that the entire battlefield becomes drenched in psychic Orkishness.

'Ere We Go, page 50