Sony has patented a game demo technology that would initially provide players with a full version of a game, but as time progresses, the game would start to degrade or deteriorate leaving players with the bare bones of the game. The full version could then be restored via an unlock code.
Siliconera who discovered the patent, gave examples of a degradable demo in that players could start with a long, powerful weapon that would gradually shrink and weaken. Alternatively, racing games could remove circuits and cars after a set timescale. Other degrading effects include dampened game visuals and sound quality.
Of course, a full game would amount to a large file size that could prompt Sony to distribute these trials on a disc, coupled with an unlock key to stop the game from degrading.