Apple Macintosh Performa 6400 Series - Manual (Page 183)

Special key combinations
If difficulties with your mouse or computer don’t allow you to use standard
methods of quitting a program or restarting your computer, you can try using
these special key combinations.
To do this … … press this key combination
Force a program to quit x -Option-Esc
Force the computer to restart x -Control–Power key
Start the computer from a CD-ROM disc C key (at startup)
Ignore the selected startup device x -Option-Shift-Delete
Turn off system extensions Shift key (while starting up)
Start the Extensions Manager Space bar (while starting up)
Rebuild the desktop Option- x (while starting up)
183 Special Keys on Your Keyboard