The shield usability pattern makes it harder to do something critical by accident. It's a protective layer that primarily consists of some sort of confirmation before the requested action is performed.
It can sometimes be very easy to do a critical action with undesirable consequences. It would be extremely bad to delete a file on the harddrive by mistake, for instance. Quitting a unsaved game because of an accidental key press is also very undesirable. In an online first person shooter game you wouldn't want to shoot your team mates, because you confuse them with the enemies. In a strategy game disbanding a unit, thus losing him forever, should be an action well thought through before executed, and not something that happens if the player clicks a wrong icon.
- There are available actions which may cause a critical, and possibly undesired, operation.
- Make it harder to do the action by accident. Add a confirmation dialog, make the user go through more menus to do the action or make the action shortcut more complex.
Some examples of shield is the confirmation dialog you get when you try to delete a file on the harddrive, or when trying to quit a running game. In the first person shooter game scenario mentioned above, you could change the crosshair icon or color when aiming at a friendly player to have the player think twice before shooting.