(Japanese) Wave of rei.
According to the 1933 book Reiki To Jinjutsu — Tomita Ryû Teate Ryôhô (Reiki and Humanitarian Work — Tomita Ryû Hands Healing) by Kaiji Tomita, a student of Mikao Usui, it describes the tingling sensation that is comparable to an electrical current. The heat created and the wave of rei are what he believed constituted spiritual energy.