Reiki is a Universal life force energy; This is the non-physical energy that animates all living things. As long as something is alive, it has life energy circulation through it and surrounding it. If your life energy is low, or if there is a restriction in its flow, you are more vulnerable to illness. When it is high, and flowing freely, you are less likely to get sick.
When a person receives a Reiki treatment, the Rei energy assesses where the person has blocks and then directs the healing energy, usually to the block that is nearest the practitioner's hands. However, sometimes it will go to the block that is most important even if it is far from the hands. The Reiki energy then works with the negative thoughts and feelings that are blocking one's natural flow of Ki and heals them as well. This can happen by flowing through the affected parts of the energy field and charging them with positive energy. Reiki raises the bivratory level in and around the physical body where the negative thoughts and feelings are attached. This causes the negative energy to break apart and fall away. Reiki will improve the results of any medical treatment, acting to reduce negative side effects, such as those from chemotherapy, surgery, and invasive procedures. it shortens healing time, reduces, or eliminates pain, reduces stress and helps create optimism. It has been regularly noted that patients receiving reiki leave the hospital earlier than those who don't.
(Rand, William Lee, Reiki "The Healing Touch; First and Second degree manual, p. 5-6, Jan 2005, Vision Publications, Hilltop St, MI)