When your tank is full, the nozzle senses this and shuts off. That's the 'Automatic' bit. You still have to press the trigger. The 'button' is just a cover for the mechanism.
But at the back of the trigger you'll see a small lever. This can lock the trigger on. You'll see a small hole, too. This is for a pin to hold the lever.
In the UK, pins are removed for safety as the petrol could continue to flow if you dropped the nozzle. Also, some people could drive off without removing the nozzle from the tank.