I must admit that I had completely forgot about Demo Play, and it’s inclusion with New Super Mario Bros. Wii. It seems that perhaps Nintendo forgot about the feature as well, because as it stands we have two different descriptions for the ‘help along’ system coming with the platform title.
During E3 earlier this year, Miyamoto spoke about Demo Play and how it would be implemented in New Super Mario Bros. Wii,
“In New Super Mario Bros. Wii, if a player is experiencing an area of difficulty, this will allow them to clear troubled areas and take over when they’re ready” confirmed Miyamoto, through his translator. “And yes, we’re looking into this for future games, too”
This description is completely different from Nintendo’s official New Super Mario Bros. Wii page: “New Super Mario Bros. Wii is also the first Nintendo game to feature a dynamic help system, which allows you to access a mode showing how a level can be completed if you are stuck. The best thing about this mode is that you are free to jump into the action you’re watching on screen at any time!”
The logical reasoning is that Demo is exactly the same as when Miyamoto described it, he just talked about it differently. As it stands now though, no one is sure if Demo Play will take the game over for you or just show you what to do.