Protecting a site from having it's database overwritten. | drupal.org
The full comment lays out the code. Pretty nifty way to prevent overwriting the live database.
Now, when @dev is used as the destination of the rsync or sql-sync command, --simulate=0 will take precedence, but when @live is the destination, then --simulate=1 will take precedence.
From: Protecting a site from having it's database overwritten. | drupal.org