Light and photobiomodulation
Nitric oxide (NO) dissociation
Nitric oxide (NO) dissociation is a proposed mechanism: that red or near infrared light knocks nitric oxide off cytochrome c oxidase, where it had been competing with oxygen and slowing respiration. Removing it would let the enzyme resume normal activity and raise ATP output.
Also called NO photodissociation, nitric oxide release, NO displacement
The proposed sequence. Nitric oxide is a normal signalling gas that can bind to cytochrome c oxidase at the same site oxygen uses, inhibiting the enzyme. The hypothesis is that a photon absorbed by CCO displaces the bound NO, restoring electron flow and increasing ATP production, while the freed NO diffuses out and causes local blood vessels to widen.
The evidence behind it. Karu, Pyatibrat and Afanasyeva (Lasers in Surgery and Medicine, 2005) added nitric oxide donors to cell suspensions before and after irradiation and found that the donors abolished the light-induced effect, which they interpreted as NO binding to cytochrome c oxidase. That is a cell-culture experiment using HeLa cells and chemical NO donors — good evidence that nitric oxide can control a light-activated cellular response.
Where it stands. It is the most cited mechanistic account in photobiomodulation, and alongside other proposals, such as light-sensitive ion channels and structured water at membranes, it remains an active area of research.
So: a proposed mechanism with real laboratory support behind it.
Why it matters here
This is the idea behind the phrase "improves circulation at the cellular level" that you will meet across red light material. It is a well-supported laboratory proposal, and interesting to understand as one.