Take a piece of tissue paper. Support the edges and press down on the center to form a bowl. The ripples that will form are extra material with nowhere to go. Now imagine the tissue was metal and you see one of the essential difficulties of putting compound curves (as opposed to a simple curve, aka a fold) into sheet metal when you need a little impression and don't have a stamping machine handy. But if you know exactly where and how to hit, you can do it with nothing more than a mallet and a tree stump.