The "Magic Sysrequest key" is Alt (left or right Alt key) and Sysrq (up there under Print Screen, next to F12 on most keyboards). To use it, you need to have it enabled in your kernel (CONFIG_MAGIC_SYSRQ). It usually is; if you have a file called '/proc/sys/kernel/sysrq' you have this. To ENABLE the magic functions, you need a "1" in that file. If it has 0, Alt-SysRq just returns you to the previous console you were using.
You do NOT want this enabled on any machine that the public can walk up to. With this enabled, pressing Alt, SysRq and another key all at the same time can do things like immediately reboot the whole box - and all it takes is the keyboard, no login required. That's all three keys held down - don't release Alt-SysRq before pressing the other key (right, it's not supposed to be easy to do)
The capabilities should be documented in "man proc", but (at least on my system) are not. The "sysrq" is mentioned, but not documented. You can find documentation in /usr/src/linux-2.4.29/Documentation/sysrq.txt if you have source installed. This is from that file:
'r' - Turns off keyboard raw mode and sets it to XLATE.
'k' - Secure Access Key (SAK) Kills all programs on the current virtual
console. NOTE: See important comments below in SAK section.
'b' - Will immediately reboot the system without syncing or unmounting
'o' - Will shut your system off (if configured and supported).
's' - Will attempt to sync all mounted filesystems.
'u' - Will attempt to remount all mounted filesystems read-only.
'p' - Will dump the current registers and flags to your console.
't' - Will dump a list of current tasks and their information to your
'm' - Will dump current memory info to your console.
'0'-'9' - Sets the console log level, controlling which kernel messages
will be printed to your console. ('0', for example would make
it so that only emergency messages like PANICs or OOPSes would
make it to your console.)
'e' - Send a SIGTERM to all processes, except for init.
'i' - Send a SIGKILL to all processes, except for init.
'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system
will be non-functional after this.)
'h' - Will display help ( actually any other key than those listed
above will display help. but 'h' is easy to remember
The key is also self-documenting: doing Alt-Sysrq-h (or any other unrecognized key) prints out a HELP message that briefly reminds you of the above functions.
If you find that holding three keys is difficult, you do have some other choices. You can trigger it manually:
echo t > /proc/sysrq-trigger
Or, you can make the Alt-SysRq "sticky" by:
echo 1 > sysrq-sticky
With that done, you can hold Alt-SysRq, release it, and then leisurely press "t" or whatever key you wanted.
You could also change the SysRq itself in /proc/sys/kernel/sysrq-key (it's normally 84 - Alt-SysRq for Intel).
If you have this enabled, it can be useful in the case where the system has escaped your control and nothing else is working. The following sequence may be better than just hitting the power button:
Alt+SysRq+s - sync the disk
Alt+SysRq+e - try to nicely kill processes
(wait a little bit here)
Alt+SysRq+i - no more mister nice guy
Alt+SysRq+u - unmount disks
(wait a bit here, too)
Alt+SysRq+b - reboot
I can remember that with "So Everything Is Unusual - Boot!"
» Magic Sysrq