What is air-time on roller coasters?
Long answer-When the G-forces are lower than +1.0. The sensation
of butterflies in your stomach is basically the effects of Gs lower
than +1.0, it literally forces your intestines upwards. Negative
force can be very dangerous because it forces blood to the brain,
rarely will a coaster ever have less than -1.5 Gs.
Short answer-When your booty leaves the seat.