The technology works on doppler shift, which is a frequency shift caused by movement - things moving away from you tend to stretch out their sound waves and things moving toward you tend to compress them, causing respective lowering and raising of the sound frequencies. A practical example is when you hear an ambulance go by with its siren blaring. At first the pitch (frequency) seems to go up (get higher) and then as it passes, it goes down.
Anyway, the article I linked to above (from the EMSResponder website) has a well written report. It is light on technical details, but it does a good job of pointing out advantages as well as potential problems