Negative G's occur when a object is
accelerating downwards at a
rate faster than that of
freefall.
This is common amongst fighter pilots taking evasive precautions in combat situations, and can produce 'redouts'. A redout happens when the blood begins to well up in the body's extremeties during a Negative G encounter. The blood supply to the head becomes swollen, and as a result, the pilot's vision will be tinted red, which can be a potentially deadly situation when depending on instruments.