Think before you sleep video

I assume she just connected opposite terminals with both cables, like how you would create an ordinary electrical circuit. That will create a very low resistance circuit between the batteries and cause an insane amount of current to start flowing.

If you look at the equation relating current, resistance, and voltage: V=IR ---> I=V/R, so low resistance at any voltage will cause a massive amount of current 'I'.

That will heat the jumper leads white hot and ruin your body work.

Conversely to actually jump a car, connect positive terminals starting with the full battery, then connect the full battery negative terminal to a ground on the chassis of the dead car again starting with the full battery. Turn on the car with the full battery and run it for a few minutes, and the turn on the car with the dead one.