How it works
When the heart is beating correctly, an electrical and chemical impulse is sent from the top of the heart to the bottom. The impulse travels the same path every time.
Sudden cardiac arrest occurs when the heart stops beating suddenly for any reason, with or without warning. One reason the heart might stop is the failure of the electrical system in the heart to conduct impulses along the correct path. Random impulses firing all over the heart will cause it to quiver uselessly, not moving blood around the body or producing a pulse.
When the AED shocks the heart it causes the heart to stop momentarily. This pause gives the heart time to reset and allows special cells in the heart to regain control and fire rhythmically, pushing blood.
One thing to remember about AED's: they only work on two specific heart arrhythmias. Only those two issues will benefit from an AED, and those are what the machine is programmed to recognize. It is perfectly normal for it not to recommend a shock and it will not save every patient in cardiac arrest. Indeed, AED's often don't help the patients they shock.
Make sure during the AED portion of your CPR class that the instructor explains the process well enough. There is an assumption - often by instructors themselves - that AED's are foolproof. Nothing could be further from the truth.
Using an AED correctly requires the user to listen carefully to the instructions coming from the machine. Pay particular attention to this part of the class; AED's save lives when they are used appropriately.