Reiki has an unusually clear history for a healing practice, because it is recent and its lineage was recorded as it went.
Mikao Usui founded a healing society in Tokyo in the 1920s and taught a method he described as received during a period of fasting and meditation on Mount Kurama. Chujiro Hayashi, a former naval officer, developed a clinical form of the practice with fixed hand positions. Hawayo Takata, a Japanese American from Hawaii, was treated at Hayashi's clinic, trained under him and brought the practice to Hawaii in the 1930s. Almost every Western lineage traces to her, and she taught a version adapted for an American audience, including an origin story that later researchers have found does not match the Japanese record.
The claim under test is that a practitioner channels an energy that promotes healing, with or without physical contact. Systematic reviews of controlled trials have not found an effect attributable to the treatment itself. The usual design compares reiki against a sham in which an untrained person performs the same movements, and the two arms have not reliably separated.
What the trials do consistently record is that people receiving it report feeling better: calmer, less anxious, in less distress. That is a real outcome, and the honest reading is that a quiet room, unhurried attention and a practitioner's hands produce it, rather than a transferred energy.
It is widely offered in hospices and hospitals on exactly that basis.