iOS 8 introduced a new way to activate Siri. With “Hey Siri” turned on (it’s off by default), any time your iPhone is plugged into power it will listen for the keywords “Hey Siri”. You can either say “Hey Siri” and wait for Siri’s beep to begin your query, or you can say it in one statement such as “Hey Siri play The Cure” or “Hey Siri tell John Smith I’ll be there in ten minutes” or “Hey Siri wake me up at 7.”
To activate go to the Settings app > General > Siri, and turn on “Hey Siri”.
This my be particularly handy (or handless, haha) in the car where you probably have it plugged in already and are within earshot.
Hey Siri only works when it’s plugged into power because having the microphone turned on and processing data constantly would drain the battery too quickly.
Occasionally Siri will think you’re talking to her when you aren’t. I’ve heard it activated when someone says “hey seriously” or when I’m listening to NPR and the reporter says “A Syrian…”