When the metal is flat, all is well. As in the tissue paper example, once a depression that constitutes a compound curve is made, the metal is going to form ripples. That extra metal needs to go somewhere. Through a specific way of supporting the work and hammering, sheet metal shapers drive the extra metal forming those ripples back into the sheet itself, making the surface area slightly smaller and the metal slightly thicker in the area being hammered. If this technique is done incorrectly, you'll simply chase the ripples all around the work piece, eventually flattening it back out, having wasted a lot of energy hammering. When it's done correctly, hammering actually makes the metal get thicker in the area being hammered, counter intuitive as that may seem.