I’m trying to figure out the why here. Do these help mask the materials from detection? Does it help amplify? I would imagine both.
Check it out:

Pressure cookers – possibly activated remotely by a cellphone – are believed to have been used to make the crude bombs that sent deadly shrapnel hurling into a crowd of onlookers and competitors at Monday’s Boston Marathon, experts told Fox News.

Doctors treating some of the 176 injured victims believe the explosives were packed with deadly shrapnel, including pellets, nails and sharp metallic objects – with some patients having “40 or more” such fragments embedded in their bodies.

“Many of them have severe wounds mostly in the lower part of their body – wounds related to the blast effect of the bomb, as well as small metallic fragments that entered their bodies – pellets, shrapnel, nails – that these bombs had,” George Velnahos, chief of trauma surgery at Massachusetts General Hospital, said Tuesday.

“I wouldn’t exclude completely the possibility that some of these fragments are environmental, but my opinion is that most of them were in the bomb,” Velnahos said.

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