I discovered this trick while repairing a MacBook Air with a broken screen. Mine additionally had a software problem that meant it wouldn’t start up far enough to recognize the display I had connected via Mini DisplayPort. To fix this, you have to:
- Power off the MacBook
- Connect an external keyboard, mouse and display
- Press the power button on the MacBook and immediately close the lid!
The MacBook will now show its EFI boot screen and all (mac)OS output on the external monitor, allowing diagnosis and software restore, as well as enabling normal usage as a “clamshell” Mac.