Although many Reiki practitioners will refer to their approach as an ancient approach it was developed less than 100 years ago. The idea behind Reiki is the use of mystical energy to perform healing, often over great distances.
Reiki master and their training should be questioned about the efficacy of their technique and the quality of their training. Being trained in the art of Reiki may often be done over the telephone as their is no real ‘technique’ and it is often just a waving of hands (and sometimes not even that!) over the client. No doubt there are many practitioners who believe in their Reiki skills, although there are probably just as many charaltans willing to take the money of the gullible.
As with many of these spiritual approaches it relies greatly on the belief of the client to create the good feelings, and as such is another example of the power of the human mind to heal itself, which is in itself the basis of the placebo effect.