In Star Trek II: The Wrath of Khan, there is a scene in which the Enterprise finds itself
Under attack from the USS Reliant, which has been commandeered by Khan.
Right at the most critical moment of the battle however,
Kirk transmits some sort of code to the Reliant, shutting down the hostile ship's shields and allowing the Enterprise to damage the Reliant and escape.
What exactly did Kirk transmit, and how did it manage to affect the Reliant so dramatically?
In addition, could any Starfleet officer with a transmitter and knowledge of the proper code do this? And if so, why isn't it ever seen again?