What toltec is saying is that the Picker button on the curves dialogue turns on click-and-drag adjustment. So once you click on the Picker button clicking and dragging your mouse on a spot in the image UP increases the Output for all pixels at the Input level of the spot you clicked on. Dragging DOWN reduces this. So, click on a shadow and drag up to brighten shadows (reducing contrast), etc. Affinity will consider the original Input value (pixel brightness value), so if you click in the exact same spot twice you will just continue to move the same dot in the curves dialogue curve spline. Note that Ctrl-z will undue all of your curve adjustments at once. If you really want to fix one dot on the curve then play with others you can click the Merge button on the curves dialogue, so that the first Ctrl-z will undo the Merge and the next Ctrl-z will undo just the last dot you created on the curve spline.