What causes a person to faint? apart from somthing simply being overwhelming, exactly how dose the body cause the person to faint or pass out or black out for a sec?

Common denominator: The common denominator is often reduced global perfusion to the brain. This is most often from a drop in blood pressure such as from hemorrhage (like a gunshot wound) or from a cardiac rhythm disturbance, sometimes with dilated blood vessels like with a simple faint. A direct CNS agent, like an anesthetic, can do it. Diffuse electrical discharge like during a seizure can also cause it.