The magic SysRq keys are key combinations within the Linux kernel that allows the user to perform various low level commands regardless of the system's state, except during kernel panics or freezes. It is often used to recover from X server freezes, or to reboot a computer without corrupting the file system.

Reboot:

Alt + SysRq + R + E + I + S + U + B

Shutdown:

Alt + SysRq + R + E + I + S + U + O

Detailed references:

• Magic SysRq keys for assistance with Ubuntu troubles - Ubuntu Forums
• Magic SysRq key - Wikipedia