The curve allows to change a parameter value by changing the curve's shape.
To change the curve's shape it is necessary to put a mouse pointer to the middle point of the curve, push the left button, and holding it, move the pointer towards the upper left or lower right corner.
Moving the cursor towards the upper left corner you will increase the parameter value, and moving it in the lower right corner you will reduce it. For example, in Local Contrast effect, moving the cursor towards the upper left corner increases contrast in the area of the preset color, and moving cursor in the lower right corner reduces it.