An Automated External Defibrillator (AED) machine shocks the heart and enables it to restart with a normal rhythm. The AED device will instruct you step-by-step, and will tell you to push a button if it determines that a shock is required.
A diagram will show you where to place the pads and when to restart compressions. If a second rescuer is present, he / she can help to attach AED pads while the first rescuer continues to perform CPR.
AED Training Video: