Positive and Negative Forces
Positive G-forces are anything that exceeds one G. This force is commonly experienced during heaving acceleration or cornering in a car as well as during certain maneuvers that fighter pilots perform. This causes the liquids in your body to move away from your head. Negative G-force is any G-force in the opposite direction of positive G-force, caused by your body accelerating towards the earth faster than the forces of gravity. This causes the liquids in your body to move away from your feet and towards your head.
While our body can tolerate positive G-forces up to a sustained nine Gs, tolerance to negative Gs is much lower. The first effect one will experience from negative Gs is a "red out." This happens because all of the blood is rushing towards your head, causing your vision to go red from the increased blood supply.
Burst Blood Vessels
If negative G-forces exceed about two or three Gs, then the increased blood in the head can cause burst blood vessels. This commonly happens in either the eyes or the brain as the vessels are overloaded by the extra blood being forced through them. This can lead to permanent blindness or brain damage.
Loss of Consciousness
Constant changes from positive G to negative G can cause an individual to lose consciousness. This is because the body's circulatory system cannot keep up with the constant changes in speed and direction of the flow of fluids, causing the heart to slow down and the individual to pass out. Once normal G-forces resume, the body will regain consciousness, albeit in a state of confusion.