You select the currently active game it seems like here:
Then it loads full screen (or unsuspends? Developer decide on fullscreen images for these scenarios?)
and end ups in the game:
While you're in the game you press the Guide button the game continues to play:
(Which allows you to do other stuff in the XB1 OS when waiting in a match making lobby for example. Assuming there aren't special cases for networking that disable this multi-tasking.)
Thanks to Benny_a, a great source of calcium and jiggaflops.
Do NOT crap up this thread like the other one with constant sniping, bickering, and stuff from other sources (like Twitter or Youtube). No tolerance in this thread.