On the new iPhone 6s and 6s Plus, the M9 motion coprocessor is build within the main A9 processor. As a result, “Hey Siri” can be always on, ready to listen to your command. With previous iPhones, “Hey Siri” is active when power is plugged in.
Sometimes, “Hey Siri“ can be activated unexpectedly in embarrassing situations. You can temporarily disable “Hey Siri” by placing your phone face down.
To switch off the always on “Hey Siri”, go to Settings > General > Siri. Simply turn off Allow “Hey Siri” switch to disable “Hey Siri”.