"When user’s HP is not full, damage +4. This is one of those skills that works for the unit. Like I said, Rinkah relies on damage output, so this skill is great...buuuuuuut, this only happens when she as at full HP, so spam healing is a must or she needs to have a high support level with the person that guard stances her, so this skill is still kind of bad. (And also enemies move second so she will most likely get damaged before this can activate, got to rely on that puppet speed)"
Not full means she shouldn't be healed if you want it to be in effect. Pretty good skill in all honesty given it's practically a free + 4 in most situations.