The
heart regulates its beating by using a
carefully-tuned
cascade of
electrical-impulses that trigger each other to keep it in a steady
rhythm, much like a
pendulum clock. When these impulses get out of
whack, like when you stick a
fork in the
toaster and subject your body to a 60Hz
alternating current, the effect is the same as shaking said pendulum clock....everything kind of
siezes up and and finds a new
stable state, which is, unfortunately, a
non-productive quivering.
A defibrillator is almost exactly like a reset switch for your heart. Applying a strong direct current zeros all unstable nerves, and allows the heart to start itself again from a base state.
When the heart is stopped completely, it is not a problem that defibrillation can solve. Instead, you need to do something like massage the heart.