I've never seen it done, just read about it. Maybe they use some kind of high temp rubber cork in each end with a clamp as well. Of course one of the corks would need a tube in it to provide the compressed air.

Here's a link that says, \"Don't try this your self,\" but there are some other good ...

There are repair shops in the US that take dents out of 2-stroke pipes. They seal off both ends of the pipe, then fill it with compressed air. Next they take a blow torch and heat the area where the dent is. The compressed air in the pipe pops the dent right out.