Many Reiki practices exist around the globe, providing both hands on services and long-distance application. This service includes long-distance application.
As with any art-form, the mindful and daily practice of Reiki will only transform ones own body to best practice the art of Reiki.
Like Ki-Gong, from Tai Ki (Tai Chi), the Ki from Rei Ki energy sympathies compassionately with the practitioner and the patient--protecting both from upsetting energy long enough for the body to heal.
Like so many energies, Ki gets it's power from the interruption of another power. In the health, the cycle of healing and sickness may strain the body and gradually eliminate the body's own power if the ill person does not carefully interrupt the sickness. When interrupted, the powers of the illness interfere with the Reiki and not the patient or the doctor. In all cases, Reiki sacrifices Reiki to allow the patient's own radiant energies to compassionately respond to ones own body--allowing healing to occur.
As with sleeping, only the restful sleep heals the best.