Reiki is not something magical. The basic principle on how Reiki energy heals is very similar to how healing works in acupuncture. We know that every living being has energy within and around the body to be alive. This energy is called Chi (vital energy) and it is vital for the physical body to exist. The physical body usually has problems because the energy does not flow properly. By stimulating the energy to flow properly as it is supposed to, the natural ability of the body to heal itself will be able to function again, and the physical body will recover. Reiki healing is able not only to stimulate the energy to flow properly but also to cleanse the energy within and around the body to speed up the healing process. In the Kirlian photos(*) below, you can see the energy around the fingertip of a patient before and after Reiki healing.
Before healing
The bright outer layer which normally acts as a protective layer (aura) is gone fading and has a lot of negativity.
After healing
The energy is cleaner and the aura is intact again.
(*) Kirlian photos are not simulation photos. The Kirlian camera has been used as radiology equipment.