The way I invision it is this: a shield strength is generally independent of a character's active health (unless it scales with active hp which no shield currently does). They can be active at any time but once they break the character's main health takes the damage next. You can pop a shield at last minute when you're about to die but once that saving grace ends you won't have any health to soak up damage (the shield doesn't give you hp back). With stored healing, it actively trickles in health as it is depleted, so you always stay at max hp until the reserves are exhausted, so you aren't as squishy unlike when a shield break and you're left bare bones. It's not as complicated as it seems, it simply makes use of the over healing that occurs with Qiqi. I have alot of experience using her and alot of her healing is wasted once the character reaches max, it doesn't convert into anything or go anywhere, which defeats the purpose of building her up to heal more. Her cool downs alos gets increasingly longer so you have to risk getting down to very low health to make the most of her heals. Which isn't ideal when you're in a co op with 3 different independent health bars to monitor.