The ELI5 version is that if the motion your eyes see doesn't match what the movement/balance sensors in your inner ear feel, your brain thinks the mismatch is b

The ELI5 version is that if the motion your eyes see doesn't match what the movement/balance sensors in your inner ear feel, your brain thinks the mismatch is because you ate something you shouldn't and begins inducing vomiting. Motion sickness. The result is that games where your real world movement matches the game world movement, such as games like Beat Saber, MOST people feel fine. But in a game where the movements don't match, such as any racing or flying game, and any game where you move by holding a button or stick, motion sickness sets in VERY quickly for most people. Lag between moving your head and moving the image creates motion sickness because it creates the mismatch. Getting head tracking latency down to zero frames was a real innovation on the Oculus Rift and is an absolute requirement for any VR headset. Secondly, the frame rate needs to be high. The original Rift was 90 fps. Valve Index is up to 144 fps. Quest 2 I THINK is designed for 80 fps IIRC. In some games, like MS Flight Simulator, it's not really possible to achieve the 144 fps my Index supports without turning the graphics wayyyy down. In this case, the software performs dynamic reprojection when I move my head. Essentially, it just moves the last frame rendered frame around the display to mimic what its movement should be. Of course, it has the side effect that the edges of vision are just black if there's no rendered data, but it's enough to prevent motion sickness when looking around the cockpit at only 40 fps. Naturally, that's moot when I actually start flying the plane. As soon as the plane starts moving, I feel queasy. But with enough practice, it can be overcome. The human brain is quite adaptive and can eventually learn to not care about the visual/physical mismatch.