What does a defibrillator do for a person?

It treats V Tach/Fib. It trys to convert v tach or v fib with pacing and electric shock in patients who are prone to these fatal rhythms.
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.