Opinion: NFC a killer app? No Flipping Chance
When detecting an RFID wristband, the reader 'agitates' a magnetic field created by a coiled antenna within the tag. The tag then uses this kinetic energy to 'power-up' and send data (held within the tags' memory), back to the reader.
The tags in RFID wristbands can either be personalised with someone's profile (e.g. data is held directly on the chip itself) or they can be used as an access 'key' to a secure database of personal data.