z_area51 is the staging area where they test out pages still in development so that is why there are no screenshots and the info is subject to change. Sometimes for whatever reason these pages are exposed to public (just google xbox fitness pass and you will find this page).
The "game" itself is not really surprising, they demoed the functionality at E3 and the xboxfitness domains made the news. I've even posted about this page a few times before (it has been up for at least a few weeks). That said, the page was updated in the last few days to indicate that it is part of a subscription service in the same vein as Xbox Music Pass and I thought that was an interesting development:
Introducing the future of fitness. Experience the world’s best workouts with instant, personalized feedback, right in your home on Xbox Fitness. Train with Jillian Michaels, Tracy Anderson, P90X, Insanity and more. Optimize every workout with immediate feedback on your form, power and heart rate with the all new Kinect, featuring Muscle Mapping, Power Gauge, and Pulse. Plus, feed your motivation with social challenges. Unlimited access is yours with an Xbox Live Gold membership through November 2014. Only on Xbox One.*
It remains to be seen how exactly it will work but if they actually provide the full p90x etc programs I might be interested. Also, lol at 2 subs required
*Some Xbox Fitness content may not be available with Xbox Live Gold, and may vary over time. After November 2014, unlimited access requires a paid Xbox Fitness Pass subscription and an Xbox Live Gold membership (both sold separately). See xbox.com/xboxfitness.
demo of the muscle mapping from e3: http://youtu.be/bydLSVVuaRM?t=7m50s
bonus: http://www.xbox.com/en-US/z_area51/hub (xbox one demo tour)