In a brief demonstration in one of its Kinect testing rooms, Microsoft showed press how the Kinect kept track of two player profiles, each tied to a controller in use. When Player 1 and Player 2 swap controllers, the Xbox One is able to recognize which profile is the new Player 1. The Kinect also monitors the position of players, meaning it can match portions of split-screen games to the side of the screen at which that player is looking. This may also translate to fighting games, which is good news if you're the sort to get confused when your spot in the couch isn't aligned with your character. [Update: The Xbox One controller itself shouldn't go without credit, as it houses an infrared LED that helps with pairing and identification.]
Microsoft also demonstrated a few more tricks made possible by the new Kinect's enhanced sense of depth, its greater field of view - which does make closer gaming in smaller apartments a more feasible – its ability to see in the dark via infrared, and its flattering scrutiny of facial features. By examining your face's skin color and transparency, the Kinect and Xbox One are able to estimate your current heart rate. Whether or not someone puts that information to good use in Kinect games or fitness programs is another matter, as we've learned from Nintendo's flatlined "vitality sensor."
Valve has experimented with biometric data in games too, adjusting game difficulty, objectives and timers in response to the player's physical state. With a Kinect shipping alongside every Xbox One, and assuming the camera is relatively accurate, biometric influence over gameplay may become less esoteric in the near future.