This is currently a cable that runs from the spectrum ST4's i/o port to the SYNC port on RED Cameras. This is used as a start stop for video shots (good for repeatable VFX shots where you don't want to touch the camera) and for timelapse shots where you want long frame intervals and to time motion control moves.
Configure your RED Brain's GPIO port as shown in the pictures. Watch out, it seems like the GPIO modes are not sticky in all cases with RED, so check this setting on power up and mode changes if you are having trouble.
On the spectrum ST4, go to Settings, I/O port, select RED Trig Cable. Now for all single pass video shots, the spectrum will trigger your RED camera to start at the beginning of the run and stop at the end of the move.
During SMS timelapse mode with the spectrum, the spectrum will trigger a Frame just like any other DSLR camera. You must make sure your RED's recording mode is Frame Recording. I suggest you set 0 Preroll and PostRoll frames.