In case anyone needed a better explanation of what REISUB stands for:
R.E.I.S.U.B.
Raising Elephants Is So Utterly Boring
R.E.I.S.U.B. is a key sequence used on Linux systems to safely reboot a frozen or unresponsive computer. It is often referred to as the “magic SysRq key” (System Request key) and allows users to perform low-level commands to safely recover or restart the system.
The acronym R.E.I.S.U.B. stands for the specific sequence of commands you can execute to reboot the system safely, even if the system is otherwise unresponsive:
R - Raw mode (take control of the keyboard back from the X server). E - End all running processes cleanly (sends SIGTERM to all processes). I - Immediately kill all processes (sends SIGKILL to all processes). S - Sync the disk (flushes data to disk to prevent data loss). U - Unmount all file systems (remounts all filesystems as read-only). B - Boot (reboots the system).
How to Use R.E.I.S.U.B. Safely
Hold Alt and SysRq, and then press each letter separately in the order of R-E-I-S-U-B. However, often the SysRq key is a dual purpose key which needs another key to activate the secondary function. For example, on a Acer keyboard SysRq is a secondary function of the Delete key and the Fn key is used to select the secondary function. So on this keyboard it would require holding down the Alt + Fn + Delete to activate the SysRq function.
In case anyone needed a better explanation of what REISUB stands for:
From http://pclosusers.com/wiki/index.php?title=REISUB_reboot_your_system>