Someone receiving healing may instantly spend a hit die to recover that many HP or may recover one hack damage. Healing may also typically be used for damage. If the target is capable of taking healing for damage (such as if they're undead), then they must roll their largest hit die and take that much holy damage. This does not consume any of their hit dice. If a creature that's unconscious receives healing, they are awakened.
Design Note: I'm open to getting rid of the Hit Dice healing limitation. It might be far too limiting. There might also be versions of it that are better. I do really want to make sure that all healing has restrictions on how much it can be done though. The main reason for the Hit Dice limit is that it was a clear, elegant, and blanket way to apply a restriction to all healing without having to rethink each individual source of it. But we might just move that direction anyways.