FAST determines if a person intends to do harm by using a mal-intent algorithm that uses data from sensors that monitor a person’s physiological and behavioral changes. FAST is designed to detect anything from changes in body movement, body heat, eye movements, breathing patterns, voice pitch, and your prosody (the tone and rhythm in which you speak).
More information (link provided by source article).
So it seems like it's a real time long range lie detector test, quite spooky if you ask me. We all know how Minority Report ended so lets hope they don't try charges based on events that were "prevented".