If you are not clear about the meanings of three squares are when you adjust Pen Pressure, please refer to the following article.
The X-axis represents the original pressure sensed by the tablet, and the Y-axis represents the adjusted pressure.
The curve shows the change of degree of input pressure and the change of degree of output pressure.
If it is a straight line, like the following image showing, it means that the pressure perceived by the tablet is the same as the output pressure. and this is the default value.
Explanation of each control point(blue, yellow and red squares)
1. The blue one cannot move up and down, only can move left and right. It indicates the initial pressure of first touching the tablet. The more it moves to the right, the higher the pressure required for first drawing
2. Yellow and red squares are the curvature of the curve controlling. The yellow one controls the former section, the red one controls the latter section and the middle part of the two sections are determined by the relative position of the yellow and the red squares.
The steeper the curve is, the more sensitive the change of pressure has. The smoother the curve is, the slower the change of pressure has.
These two squares control three segment curves.