@sirflimflam
In chapter 1.1, they explained it as the healer needing a full understanding of the target in order to literally recreate every experience that the body has gone through so as to do a complete treatment of the body. Everything the target of the heal has gone through will be experienced by the healer. All the pain, anguish, and experiences are transferred to the healer. That was called the fatal flaw of healing. He retains a portion of the experience and skills of everyone he heals.
In chapter 1.2, he explains that with recovery heal he is basically returning someone's body to an earlier healthy "state." With simulation heal he can copy the "state" of someone he's healing. With upgrade heal he can choose the specific desired state of a target. With corruption heal he can choose a corrupted state of a target. With time reversal heal he can rewind the entire world to a specific state. It's a stretch, but the author found an interesting way of spinning things.
Also, in chapter 4.1, Frey's henchman says that his recovery heal is different than a normal person's recovery heal. His heal is in a sense "god level" allowing him to create from nothingness or even turn back time.