G-force is used to describe the acceleration of an object relative to Earth's gravity,
Humans adapted to survive in a 1 G environment, 3 G's can be enough to deprive the brain of oxygen.
When a plane makes a rapid, near-90 degree, it creates a radial acceleration that can generate more than 6 G's of force — or six times the force of Earth's gravity.
The acceleration force is strong enough to force the blood down the legs, making it difficult or almost impossible to flow back to the heart for re-circulation, brain will "shut down" causing lose consciousness .
