@tito_swineflu @it4sec I would assume with opening the door the car sends a "key are you near?" signal. They probably use antenna to transmit the signal further and catch the answer from the key back to the car.
Keys today have protection that you can not randomly get a code and use that on the car, I think. They need to follow such a handshake protocol, so no one can easily duplicate the key. So instead (I guess) they extend the range of the signals.