Xbox One includes an upgraded Kinect sensor that can not only recognize your face, but your voice as well. In fact, the new Kinect will be able to understand even when two people are speaking simultaneously. "We are focused on ensuring that we have great voice support which includes understanding which person is speaking in the room," Microsoft's Marc Whitten explained.
It seems likely that voice recognition is being paired with video information from the camera, as Polygon points out that Kinect can identify when people's mouths are moving--even when in a dark room.
Of course, what do you do if your friend is trying to grief you? Well, those audio commands will require a further confirmation, as in the case of "Xbox Off," Whitten explained.
If you don't want to deal with Kinect at all, however, Microsoft does remind gamers that it can be turned off. "You have the ability to completely turn the sensor off in your settings. When in this mode, the sensor is not collecting any information," Whitten told IGN. However, you have the option of having Kinect turn on automatically by playing a Kinect-required game. "If you enter into a required Kinect experience (like Kinect Sports Rivals for instance), you'll get a message asking if you want to turn the sensor back on in order to continue." This is perhaps the best option for gamers that don't want Kinect to be on--but want to still play games with it.