This high G-force can push heads down and have blood rush from your brain down to your feet, which in turn lowers the oxygen level in your brain, which may lead to gray outs, loss of peripheral vision (known as tunnel vision), or temporary blindness.

Passing out during high G-force maneuvers (specifically "positive Gs" or +Gz​) is caused by cerebral hypoxia, which is a lack of oxygenated blood reaching the brain. When a roller coaster or fighter jet pulls a sharp upward turn, the centrifugal force "pulls" the blood away from the head and toward the lower extremities (the feet and legs). The heart must work much harder to pump blood vertically against this increased force; if the G-load is high enough, the blood pressure at the brain level drops to zero. This leads to a progression of symptoms: first, "greyout" (loss of color vision), then "blackout" (total loss of vision while still conscious), and finally G-LOC (G-induced Loss of Consciousness). In 2026, modern fighter pilots use G-suits that physically squeeze the legs to keep blood in the upper body, but for the average person on a high-G roller coaster, the best defense is the "anti-G straining maneuver," which involves tensing the leg and abdominal muscles and performing short, sharp "hook" breaths to maintain internal blood pressure.