I may be misunderstanding what you are talking about, but growing up in a bodyshop when you could push the metal in and it would stay, and then get it to pop out and it would stay, the metal was stretched and needed shrunk. There was three was I remember to do this, one was a brazing tip and a bucket of cold water with a sponge. You would heat a small circle pretty hot and then throw the cold water on it and strink it until the metal stayed stiff. The second was taking a body hammer and using the sharp point to make a bunch of tiny little dimples that will take up the slack of the stretched metal. The third is similar to the first, you use a dent puller that welds the studs and it has a fitting for it to shrink the metal.
