If you have an old school ignition system with mechanical points and capacitor sometimes the engine speed reading can be a bit jerky due to oscillations between the ignition coil and capacitor. This cable will in many cases solve this issue if it remains after a new set of points and new capacitor has been fitted.
This cable is only needed on OBX version 1.1.1 (in the case of jerky engine speed signal) as OBDX 2.1.1 has the feature included in the hardware.