How Does "Hey Siri!" Work Without Your iPhone Listening To You At All Times?

“Hey Siri!” feature was launched in iOS 8 in September 2014 as an additional feature to the pre-installed voice assistant- Siri. In iOS 9 (September 2015) it was upgraded with the capability to identify only the personalized voice of the user. Even when your phone is idle, the coprocessor is listening(not hearing) to every sound that falls on the mic. This is of great benefit as the user can access the basic features while physically being engaged and is even helpful in an emergency situation.