The most interesting thing to me about Reiki is that nobody really knows what it is and what it does. We can guesstimate through experimentation I suppose, what it does. People report feeling calmness, relaxation, falling asleep during a seesion, feeling more energized after a treatment, pain reduction, inflammation reduction, etc. There are lots of theories about it's origins, the main one being the Christianized version created by Mrs. Takata that was debunked after her death for lack of evidence, or evidence that absolutely went counter to what she told everyone, like no record of Usui attending seminary school at the University of Chicago, and the fact that he was raised in a Buddhist monastery and even became a lay Buddhist Priest.
There is even talk of Usui not having used or created any Reiki symbols, those being the product of Dr. Hayashi and the other Naval officers who, in Reiki circles is accepted to be the heir if you will of the Reiki world after Usui's untimely death at 54 of a stroke. Also that Usui didn't actually have the big bang moment on Mt. Kurama but actually performed The Lotus Repentance Meditation (21 day fast and medittation on the sacred mountain for enlightenment) many times and that actually Reiki is a compliation of all the modalities systems he studied.
Do we really need to know what it is or what it's origins are or can we just accept that that is astounding, miraculous, amazing and so loving and gentle? Probably not because our egos need answers and so the search goes on.