A holistic perspective sees health as a dynamic state of balance with considerable resilience in the overall system. A healthy person can withstand a certain amount of stress and bounce back. This is because the human body's ability to heal includes various self-regulating mechanisms that maintain overall balance, which is known as homeostasis.
However, after experiencing sudden, intense stress  or moderate stress that is constant over a period of time, the body's ability to regulate itself becomes compromised and health declines, slowly or not so slowly. Stress causes the body to lose its ability to rebalance, to restore homeostasis.
Unless they are addressed, the stresses of everyday life - environmental , emotional , physical, financial , social etc. - can combine with an individual's genetic predispositions and result in declining health and wellbeing.
At the simplest, most obvious level, it appears that Reiki  treatment helps lessen the impact of stress, releasing tension from the entire system. Not only does the person move toward his or her own unique balance in body, mind and spirit, but also, depending on the level of physical health when Reiki begins, the body's own healing mechanisms often begin functioning more effectively.
How exactly does Reiki release tension and help the body heal? That question has yet to be answered. Although there is increasing research evidence documenting the effects of Reiki (such as lowered heart rate, blood pressure, and stress hormones; increased immune strength), we have only broad theories as to what causes these effects or the pathways through which the healing occurs.
The multileveled, rapid response to Reiki suggests a complex process that engages many body systems, simultaneously or in quick succession, shifting the body from domination by the "fight or flight" (stress) response to the relaxation response, and supporting the body's own healing mechanisms. (For more information about the relaxation response, see the Mind-Body Therapies  section.) Some researchers theorize that Reiki's physical, mental, emotional, and spiritual healing effect is triggered on a subphysical level, perhaps in what science refers to as the biofield.
The biofield is the term medical science has adopted for the intricately layered vibrational energy field that is said to surround and penetrate the physical body. The biofield is extremely subtle and there is as yet no conventional scientific technology that can document its existence or otherwise study it directly. However, for thousands of years, traditional, indigenous, prescientific medical systems have recognized a balanced, evenly pulsing biofield as the foundation of health and wellbeing, and seen any disruption of this balance as the beginning of illness.
The healing traditions of indigenous cultures around the world use vibration to restore balance through ceremonial drumming, through instruments such as the didgeridoo and the tamboura, and through chanting, overtoning, and humming. Moreover, the therapeutic value of vibration in the forms of sound and music is increasingly supported by science. Perhaps Reiki's benefits occur through the same vibrational mechanism, a process which likely involves increasing coherence and decreasing dissonance in the system.
One theory is that the Reiki practitioner's hands carry the energetic vibrations of the recipient's wellness. The effect of Reiki could be seen as a shift in attention in which the recipient becomes aware of the wellness that exists deep within his or her being regardless the current level of health. Reiki could then be understood as entraining the recipient back to health in a process similar to the way grandfather clocks in the same room self-adjust to the rhythm of the dominant clock, or in the way we relax when in the presence of someone who is deeply peaceful. Reiki seems to connect the practitioner with an inner source of peace no matter how he or she feels in the moment.
Many therapies aim to restore balance in the biofield-acupuncture , qigong , shiatsu , and yoga /pranayama to name a few. Reiki appears to be the subtlest of these therapies, balancing the recipient through subtle Reiki vibrations rather than through manipulation or even the gentlest force. It is also possible that Reiki, which some practitioners consider more akin to meditation  than to energy therapies, does not originate in the biofield, but rather arises from an even subtler source: what physicists have termed the unified field.