How does this work for Hilton's that are sold out? Does someone at the bottom of the totem pole lose their room? Is it always guaranteed that a room will be available?

If you're a big college football fan, you'll learn that many hotels are sold out months in advanced for some of the smaller towns, and often times you can't find a decent hotel within an hour's drive. If the 48 hour guarantee is true, can I simply decide on a whim I want a room in a prime location and boot some poor soul who's had it locked up 5 months in advance?