Is the rule mentioned in the skull pass set? (I don't have it so don't know) Otherwise it is probably described in the orc and goblin book unless GW changed the rule.
Woutah, we are not allowed to give away rules or points values on the board since it breaches GW intellectual property. We can point you to where to find rules though, so I'm leaving this open for someone who knows where the rule is to give a page reference.