Maybe you're right and it's for jump starting, just drop the black lead on the ground. Then hook up the positive to the rubber (which has a conductor inside it that none of us have picked up on) and start your car, it means you can rip off the positive terminal from the drivers seat as soon as the engine kicks over. Once the lead is off you can just throw it at the car helping you out, don't worry about sparks or anything cause the black lead will still be earthed.