A more scientific explanation. It's all about vibration and the relaxation response. We need energy to flow freely through our bodies to keep our cells vibrating at a healthy frequency. When we block our energy channels from of trauma, stress and negative emotions, our cells start to vibrate at a lower frequency. This will cause cellular breakdown. The other component is the relaxation response. Reiki helps the reiki client become extremely relaxed through entrainment. Entrainment is when the clients energy matches the calm energy of the practitioner. When the client becomes relaxed the tension in their body eases. When this happens the energy starts to flow and balance. This causes the stress response to lower which causes the immune system to function properly. The immune system is what heals our bodies. When you have both physiological states in balance, healing is possible. So here's another way to explain it to those that think reiki is to woo woo.