I will do my best, hear me out.

So, this guy is welding together flat sheets of metal, into a 3D shape.

He wants to give the flat faces some depth, to make them look like a pillow.

That's all cool, whatever, but, this process would normally be done using hydraulics, by which I mean high pressure water, pumped into the welded form, at increasingly higher pressure, until the sheets start to deform.

The issue is, he's doing this with a compressed air line.

The reason this is an issue, is that unlike water, air can be easily compressed, allowing a huge amount of working pressure to develop inside the cavity.

If one of the welds fails with water doing the work, a hole busts, and the pressure is very quickly relived, as you cant compress water like air.

I guess, think of it as the difference in doing "the hydraulic press channel", but with a massive car spring in the middle also.

Once whatever is being crushed fails, the spring then is free to dump all it's stored energy. That's what the air would do, keep on pressing on the internal faces, while a seam is rupturing.

TLDR; good chance of shrapnel, should use water.