Rhythm restoration. Assuming the problem is ventricular fibrillation, the chaotic and fatal cardiac arrhythmia, one must know there are hierarchies of pacemakers controlling the heart rhythm (fastest one usually wins). By shocking the heart into functional standstill, the next available fastest stable pacemaker tissue takes over, usually sinus node > ectopic atrial > junctional > idioventricular/purkinje.
Defibrillators. The device has an antenna that monitors the electrical activity of the heart. If the recorded parameters fit those of ventricular tachycardia or ventricular fibrillation, the device will charge its capacitor and deliver programmed shocks to try and return the rhythm to normal. It is a computer and programmed to do its functions. They are very effective but not perfect.