It essentially details a taster for a full game, in which elements are taken away depending on how far you've progressed or how long you've been playing. Those elements can then be restored, should the user decide to purchase the game.
In essence, these demos would allow players to experience complete games before purchase. In comparison, current demos only provide a particular slice depending on what the developer wants you to see.
There are specific examples of this idea over at SiliconEra, including weapon deformation and the removal of tracks from a racing game.