Semi-automatic defibrillators will ask the rescuer to press the shock button to deliver the shock allowing time for people to stand back from the patient when the shock is delivered. It also allows you to press the shock button as quickly as possible so you can start compressions again.
A fully automatic defibrillator will automatically do the shock for you, with no need to press the shock button. Whether your defibrillator is a semi-automatic or fully automatic model it will always make the decision to shock if it is required.