When cast, this spell allowed the shugenja or targeted creature to gain an immunity to all non-magical weapons. A powerful shugenja was able to use this spell to grant immunity to some magical weapons as well. Once cast, the recipient was immune to damage caused by a weapon. This immunity only applied to weapons, the recipient was still vulnerable to natural abilities, such as fangs or claws. They were also vulnerable to weapons wielded by very powerful creatures.
Vulnerability was the reverse of the spell. It had the opposite effect, lowering a targeted creature's tolerance for damage. It also allowed a creature who was immune to non-magical weapons to become vulnerable to such as attack.