What does it mean when TSA wipe your hands?

What does it mean when TSA wipe your hands? Wait, what? TSA officers swab your hands with a cotton cloth to collect explosives residue for testing in an Ion-Mobility Spectrometer (IMS), the machine they put the cloth in that determines if you go to your gate or to a private security screening.

Wait, what? TSA officers swab your hands with a cotton cloth to collect explosives residue for testing in an Ion-Mobility Spectrometer (IMS), the machine they put the cloth in that determines if you go to your gate or to a private security screening.