Using Siri in Handsfree Mode
From iOS8 upwards, you can enable Siri’s new handsfree
You can speak to Siri without pressing the home button by saying ‘Hey Siri’ when connected to a power supply.
‘Hey, Siri’ works in any situation as long as the iPhone or iPad is plugged in, even if the screen is locked or if an application is open and running. Users can speak naturally, and do not need to pause after saying ‘Hey, Siri’.