Nothing special to add about the magic SysRq key than http://en.wikipedia.org/wiki/Magic_SysRq_key.
According to the mentioned link above "the magic SysRq key is a key combination understood by the Linux kernel, which allows the user to perform various low-level commands regardless of the system's state. It is often used to recover from freezes, or to reboot a computer without corrupting the filesystem".
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