"As the lender, you may always access and play your games at any time. If you decide to start playing when a friend is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing."
So I can authorise a person to use my account. I can then, at any time, play my own games. But if that other person is playing a game it will kick them off.
I am really excited about this feature. Around the time Portal 2 came out my partner and I lamented the fact that we had to buy two copies if we each wanted to play on our own account, considering how account sharing is against the TOS.
I'm sure the people who do account share to get around this will be very happy to know that they don't have to go over and log in their significant other whenever they wish to play a game the other owns.