Air time has a strange effect on your body because your body is not completely solid — it is composed of many parts. When your body is accelerated, each part of your body accelerates individually. The seat pushes on your back, the muscles in your back push on some of your organs and those organs push on other organs.

The "stomach-drop" sensation you feel on a roller coaster is technically caused by airtime or "negative G-forces," but the physiological feeling itself is related to something called visceral displacement. When a coaster drops suddenly, your body accelerates downward faster than the organs inside you. Since your internal organs (like your stomach and intestines) are somewhat "loose" and suspended by connective tissues and mesentery, they actually lift up slightly inside your abdominal cavity while the rest of your body—anchored by your skeleton and the coaster seat—falls away from them. Your nervous system detects this sudden shift in the position of your "insides," which creates that fluttering, "weightless" feeling. This is essentially a mild version of what astronauts feel in space. While many people describe it as "butterflies," it is actually your body's vestibular system (inner ear) and proprioception (sense of body position) reacting to a brief moment of freefall. For some, this sensation triggers a rush of adrenaline and joy, while for others, it can lead to motion sickness or anxiety.