Page 4 wrote:Mortal Wounds
Some attacks inflict mortal wounds. Do not make hit, wound, or save rolls for a mortal wound - just allocate the wounds to models from the target unit as described above.
So it can essentially be read as generating a mortal wound on a roll of 6+ at the cost of the attack itself. Stop processing the attack and instead inflict a mortal wound (which bypasses to hit, to wound, save, and damage).