[bioguy (OP)]

What is happening in the body in sudden cardiac arrest? What does an AED actually do when it shocks the body/heart?

[Janus]

What is happening in the body in sudden cardiac arrest? What does an AED actually do when it shocks the body/heart?

A defibrillator can't restart a fully stopped (flat line) heart.  What it does is get a heart which is undergoing ventricular fibrillation, which is a condition where the ventricles of the heart are not contracting properly, back to a normal operation.
The shock depolarizes the muscle which allows the mechanism that controls the heart beat to regain control.
The importance is of this is that  ventricular fibrillation will lead to full on cardiac arrest, and a stopping of the heart if not treated.
An AED reads the heart rate and determines whether defibrillation is the proper course of action.  Thus the device itself determines whether or not a shock should be applied.