A defibrillator **isn't** used to restart a heart that has stopped beating. It is used to stop the fibrillation of the heart and will "reset" it to its normal rhythm. Thanks to TV shows and movies a *lot* of people have this misconception about what the device will do and what it's meant for.
Also, when the shock is administered the patient won't spasm violently. The muscles in the chest area will contract, but it's nowhere near as violent as often portrayed.