1. It's easier to understand if you read the TIPS in "Understand with 3 frames! Level correction" first.
This is a function of the combined type of "output" and "input" explained at the time of level correction.
2. "Input" on the horizontal scale indicates the darkness of the screen color. The tone curve is used to "output" the color by finely setting the desired brightness on the vertical scale. It's hard to understand with only numbers, so I put a brightness bar.
3. Brighten the dark color of "input" and "output". "Input" bright colors are darkened and "output". Less contrast.
4. If you set the darkest part to the maximum brightness and the brightest part to the maximum darkness, it will be reversed.