By communication with navigation satellites, detailed real-time data on flight variables can be passed to a server on the ground.
They are typically placed inside of the airframe in a location where the GPS and communication satellites are directly visible to the device, for example through the cockpit window.
Authorities classify non-installed components as "transmitting portable electronic devices" (T-PEDS) and as such require them to be switched off during the critical phases of flight.
[1] Accurate real-time data provided by GPS aircraft tracking may be offered to air traffic control using ADS-B technology.
If the unit stops transmitting upon impact, the historical transmissions will give the last known location of the aircraft, its speed, direction and altitude.