Share the damage between the caster and another.
Caster shares all damage received with a willing target. This means that both receive half of the damage that the other receives for the duration of the spell. So if one takes 10 damage, they will both take 5 damage.
Through enhancing, it is possible to lower the shared damage received, but not the initial damage. If one takes 10 damage, he or she will still take 5 damage, but the other will take 4 or even lower.