But in the state of free-fall while plunging down a hill or a roller coaster's curve, there isn't net force to act upon. Various organs are essentially falling at the same time, but individually inside of your body, they are weightless. Your stomach is suddenly very light because there is less force pushing against it.

The "stomach drop" sensation on rides like Disney's Tower of Terror or Cedar Point's Power Tower is caused by a physiological phenomenon known as visual-vestibular conflict, combined with the physical displacement of your internal organs. When a ride suddenly drops, your body enters a state of free-fall, which momentarily reduces the normal upward force of your chair against your body. However, your internal organs (like your stomach and intestines) are not rigidly attached to your skeleton; they are suspended by ligaments and mesentery. As the rest of your body accelerates downward at 9.8 m/s2 (or faster if the ride is "pulled" down), your organs slightly "float" inside your abdominal cavity due to inertia. This creates a brief moment of negative G-force. Your brain receives a high-fidelity alarm signal from your vestibular system (inner ear) and your nervous system, interpreting this sudden lack of internal pressure as a "drop." While it feels like your stomach is moving toward your throat, it is actually just a momentary lag in acceleration compared to the rest of your body's descent.