In GIMP and Photoshop the Curves graph can be manipulated directly, but clicking on the diagonal line and dragging points on the line into new positions, or using the arrow keys to nudge the selected point up, down, left or right. There's no limit to when the line can start or finish, or where it goes in between, which can lead to some extraordinary results as we'll see. Unfortunately Photoshop Elements doesn't provide quite as much freedom to experiment; pre-sets can be applied to the Curve, and adjustments can be made via sliders, but only within defined parameters.
One parameter that is allowed is contrast adjustment. This changes the shape of the diagonal line into a narrow S-shaped curve, slightly raising the upper half of the curve and lowering the lower half of it, leaving the mid-point and both ends in the same positions. This has the effect of brightening the lighter areas of the image, while darkening the darker areas, increasing the overall contrast while maintaining the same overall exposure. It's an excellent way to make very precise adjustments to contrast. Mid-tone contrast can be adjusted by moving the mid-point on the curve to the left or right, while overall brightness can be changed by moving the mid-point up or down.