Reiki works the sub atomic level to ferret out areas of problems (mental, emotional, physical, and spiritual) within the human body as a whole. Practitioners often describe these issues as dis-ease, energy stagnations, blockages, and scar tissue. The best way to describe the process is that to the Reiki energy these unhealthy forms are magnetic. Just as a magnet is attracted to larger more dense metals, Reiki is most attracted to the bigger, more entrenched imbalances in the system. In this way, healing energy is focused where it is most needed.
Reiki is a form of quantifiable energy with particular wavelengths and characteristics. Thus, Reiki is just like other energy waves that we are already familiar with like light and sound. Humans can only see a very limited range of frequencies within the visible light spectrum. We can physically perceive a slightly wider range of wavelengths through heat (ultraviolet light) or sound (radio and sound waves). Other wavelength frequencies such as gamma rays, x-rays, infrared light, and subsonic sounds are beyond the natural human ability to see, hear, or otherwise perceive.
To the naked eye, Reiki is invisible. That is, the vibrations of this form of energy are outside the visible light range. However, Reiki is perceivable, mostly as variations in heat and cold. This perception is often apparent to both the practitioner and the individual receiving the energy.
Beyond its magnetic attraction to dis-ease within the body system and its ability to produce detectable temperature changes on human skin, Reiki has one more important characteristic. This one has direct applications for healing. Much like a laser or ultrasound, Reiki hones in on a particular problem area and cuts, breaks up, and dissolves the issue.
For even more great posts about Reiki, you may want to check out Reiki Basics, Reiki Healing, The Nature of Reiki Energies, Learning Reiki, and Reiki Best Practices. You can also get a Free Visionary Reiki Session.