They are used to start and stop a track from a player that is capable of using fader start like most of the CDJ line. When you open the fader the track starts when you close the fader the track re-cues and pauses. You also have to have the control cables connected between the decks and the back of the DJM
basic video on it: http://www.youtube.com/watch?v=7WWhb0EW-As