Here's why:
* Save files are written in stages: When you save in Pokemon Diamond, the game doesn't save everything at once. It writes data to the save file in stages.
* Interruption corrupts data: If you power off the DS during this process, the save file may be incomplete and corrupted. This can lead to:
* Lost progress: You might lose all your progress since your last save.
* Corrupted game data: The game might become unplayable with glitches, errors, or even a completely blank save file.
* Brick your game: In rare cases, severe corruption can render your game cartridge unusable.
What you should do instead:
* Always save in a safe place: Save your game in a place where you won't be interrupted, like in your house or a safe location in the game.
* Wait for the save to complete: Don't touch the DS or power it off until you see the "Save Complete" message on the screen.
* Use a backup: If you're worried about losing your progress, use a backup save file on a separate DS cart or a computer.
It's always best to play it safe and avoid powering off the DS during the save process.