Water expands when it freezes. You can use that property to your benefit if you're trying to get the dents out of thin metal pipes or dented metal containers or -- in the case of this particular Instructable -- a brass instrument.
While the ice trick was meant specifically to repair Instructables user swander's French Horn, you could use it to fix a dented metal pipe or tube, a busted bucket, or a metal tank, as long as the metal is thin enough to be pressed out from the inside. All you have to do is fill up the container with water, seal off the ends, and put it in the freezer. Swander says he sealed the ends of his horn's leadpipe with the fingers of a latex glove and pipe clamps so the freezing water could expand into the latex and wouldn't burst the leadpipe. It worked and his leadpipe is no longer dented.
If you're trying this with a pipe or other sealed container, you'll want to make sure you get a complete but flexible seal on all of the exits, but if you seal it too tightly you may rupture the container. Ultimately though, it's easier to keep re-freezing the container than replace it if it ruptures, so err on the cautious side if you try repairing an old gas tank or water drum with this trick.
Brass Horn Repair with Ice [Instructables]