I'm the owner of the account, so I share my entire library with a friend, Player A.
Player A can now play any game I own, providing DRM doesn't say no, in which case they wouldn't see it.
Player A starts playing Worms, I want to play, so I load Worms too, they get 5 minutes before the get kicked off, or buy the game to continue playing.
Owner is playing Worms, Player A can't boot it as Owner takes precedent.
So far, all as we thought it would be...Then they hit you with the fun stuff.
You can only play ONE game PER LIBRARY.
Player A wants to play Worms, I come off it so they can. They're playing happily, I decide I want to play Shadow Warrior, different game, all should be fine.
5 minutes later Player A has been kicked out.