H Amy,
The tracker is an electronic device that was inserted under the skin of each tribute's forearm as they boarded the hovercraft to go to the arena. It tracks location and physiological condition. If body function falls below a preset set of norms the signal it sends back indicates that the tribute is physiologicaly 'dead'. Those physiological markers were set by physicians based on what they predetermined the readings would be for one who is dead.
In most cases that is cut and dried. Heart stops = person is dead.
