Magic system recovery keys are combination of key hits that we can perform and kernel responds on priority. Before using magic keys we need to enable CONFIG_MAGIC_SYSRQ, by default sysreq compiled file (/proc/sys/kernel/sysrq) contains 1 that enables every possible sysreq.
To customize this file we can use command :
echo "integer" >/proc/sys/kernel/sysrq
integer 0 disable sysreq
integer 1 enables all functions of sysreq
Using:
echo command > /proc/sysrq-trigger
command:
b : immediate reboot without unmunting.
c : perform system crash by null pointer de-referencing.
d : shows all locks that are held.
Rebooting your system due to freeze state (hanged state) may
temper your file system.We can use magic key controls to handle
the situation that was initially used for kernel debugging.
Securely Restarting Hanged System:
Hit ALT+PrintScreen While holding the above keys press following given below
keys one after other (press and hold for 3-4 seconds)
For Reboot:
R S E I U B For Shut Down:
R S E I U O What it does? : 1. It takes control of keyboard back from X2. Sends SIGTERM to all processes, allowing then to terminate racefully.
3. SIGKILL to force all processes terminate immediately.
4. Flushes data to disk.
5. Remounts or Unmounts all file systems.
For more details please refer:http://en.wikipedia.org/wiki/Magic_SysRq_key 
