Are you talking about voice control? Google Voice is a system by which you get a second phone number and you can get texts, voicemail and phone calls over data rather than using minutes or text messages off your plan (http://voice.google.com)
On the S6 you have S-Voice (if it's still around) and Google Now (which can respond to voice commands) for voice control. I don't believe there is a voice command that will turn off the device.