First of all it applies a huge heal over time to the target, but if they die while this heal over time is in effect it'll prevent their death and heal them for a flat amount and dispel itself.[1] – John Collins
Heal target ally for 50% of their maximum health over 10 seconds. If that target would receive fatal damage while under this effect, this effect is consumed and prevents that target's death, healing them for 25% of their maximum health instead.[1][2]