What is Resonance?
Resonance is the phenomenon of "ringing" a harmonic system through timed reinforcement (e.g. ringing a bell, pushing a swing). When a vibrating system is stimulated in just the right way, a large and focused output of energy is created for a relatively small input, due to overlapping wave reinforcement. In Physics it's defined as "a vibration of large amplitude produced by a relatively small input near the same rhythm as the natural harmonic frequency of the resonating system." At resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations, due to the storage of vibrational energy.
The neurovascular baroreflex (blood pressure sensing reflex involving blood vessels and nerves) has a resonant frequency of approximately 6 cycles per minute (or ~0.1 Hz). The exact frequency is unique to each person, and thought to be proportional to the total blood volume. This frequency typically lies within the range of 4.5 to 6.5 breaths per minute (or 0.075 to 0.13 Hz).
How does deep breathing affect the body?
We all know that breathing creates the exchange of oxygen and carbon dioxide in the lungs and blood, however most people are not aware that breathing also circulates the lymph. The intricate function of the lymphatic system is a relatively modern discovery, as it couldn't be completely mapped with the naked eye. While blood comes and goes via the arteries and veins, lymph only has a one-way path: between every cell and back up into the heart.
Blood plasma leaks out from capillaries delivering oxygen, nutrients and hormones to the cells, which in turn release waste and signaling chemicals. 90% of this extracellular fluid goes back into the veins, but 10% goes into lymphatic capillaries, beginning a one-way trip back to the heart, via a network of vessels, nodes, and ducts. The final tube that lymph collects in before it goes up into the heart to mix again with the blood, is called the thoracic duct.
The thoracic duct is is sort of like a turkey baster; each time a deep breath is taken, it gets squeezed and vacuums the lymph up from all of the interstitial space (between the cells).
How can just one exercise as simple as resonance breathing be so effective?
Resonance breathing amplifies the intercommunication between crucial parts of the body and brain, creating a harmonious dialogue between the intelligences of the belly, heart, and head. It engenders a sense of safety which allows the human spirit to blossom (think Maslow's hierarchy).
It is also proposed by skilled manual therapists and osteopaths that the resonant frequency of the lymphatic system is identical to the baroreflex resonant frequency. This makes sense because the ocean of salty water in the body exchanges freely between the cerebrospinal fluid, lymph, and fascial microtubules.