Pardon my basic terminology but it's what happens when the metal gets stretched from damage and loses it's "tin" (I believe that's the term). The metal gets to a point where you can push it in and it pops back. Similar to when you open a jar of jelly, etc that has been vacuum packed. Before it's ever been opened, if you push on the center of the top you can't push it in. After it's been opened you can press down on it and it pops back. It gets the name from the cans oil used to come in before the plastic bottles. Make sense?