Another really interesting post from TSA Blog: Evolution of Security: (which should be called 'The TSA Blog'!
"Behavior analysis is based on the fear of being discovered. People who are trying to get away with something display signs of stress through involuntary physical and physiological behaviors. Whether someone’s trying to sneak through that excellent stone ground mustard they bought on vacation, a knife, or a bomb, behavior detection officers like me are trained to spot certain suspicious behaviors out of the crowd. Once we make our determination, we refer these passengers for additional screening or directly to law enforcement."