The transmitter of the student (trainer transmitter or buddy box) is connected by a trainer cable to another transmitter (master transmitter) controlled by an instructor. This creates the buddy box system. The instructor is able to control the RC with his or her transmitter then flip a switch or press a button to turn control over to the student and flip the switch again (or release the button) to reclaim control. In this way, the instructor can easily demonstrate how to fly the RC or rescue the RC if the student makes a bad move without passing the same transmitter back and forth (which would likely result in delays and crashes).
Setting Up a Buddy Box SystemTo create a buddy box system you'll need a trainer cable (also called a buddy box transmitter cord) and two compatible transmitters that will work with that cable. Futaba, Spektrum, Hanger 9, and others make trainer cables for a buddy box system. Some are designed to work with specific transmitters. Although both transmitters must be compatible with the trainer cable they do not have to be identical transmitters.
The master transmitter is the one used by the instructor. It is the only transmitter actually communicating with the RC aircraft. The trainer transmitter sends signals through the trainer cable to the master transmitter. It doesn't even need to have a crystal since all radio transmissions to the receiver in the RC come through the master transmitter. Some trainer transmitters are designed only for use in a buddy box system and won't work as an independent transmitter.