Defribrillators work by sending an electrical shock to the heart via the pads to re-commence a heart beat and heart rate. They do work alongside with drugs like adrenaline which stimulates the heart. They can and do re-commence the pumping action of a heart and can also cause the heart to stop if thye are used on a person whos heart is already beating.

A paramedic came into my career class back when I was in gr 10. And he said most of the time, the defib's just kill the patient, but it's just sort of a last resort, because they are will die anyways