You can use it when you have enough HP to "sacrify", I mean it doesn't consumes MP but HP.
With Sacrifice skill you heal your target, that means you can heal anyone even yourself, but when you use this skill, it takes away a fixed amount of your HP. So, if you use it on yourself, you'll lose HP (skill requirement) and then be healed (skill effect). If you cast it on anyother target you will only lose HP (ppl use this skill in pvp servers to get hp into 30% and then activate AI)
if you don't have any buff that increases m.attack or increases the healing you recieve, the HP restored should be almost (if not equal) to skill's power.
If you want to use it to heal yourself, you have to increase your healing power, m.attack... and ofc cast it on yourself
P.D.: maybe you wanted to know about "Touch of Life"
