fixing dented headers

[vinnY]

http://www.thumpertalk.com/forum/sho...d.php?t=523620
it appears freezing water in dented headers can pop out the dents, anyone happen to know why this happens?
if they totally filled the header with water i'd understand but in the article it states:

Added enough water to fill the dented area of the pipe, plus a bit more (did not end cap the pipe or plug anything).

which sort of boggles me as to how it'd make the dent pop out if it(the water) has space to expand into

anyone know any other methods of getting dents and dings out?

looking to fix my tri-y's from all the battle scars