Undershirts and underwear have the 4th amendment written in metallic inc, which will show up in an x-ray.
The fourth amendment of the Constitution of the United States reads:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
The website says that with their products, you can “protest those intrusive TSA X-ray scanners” and “Assert your rights without saying a word.”
Forth Amendment Wear comes in T-shirts, undershorts, and bras, so the TSA will see that you do not approve of their unwarranted searches.