kernel.sysrq

/usr/src/linux/Documentation/Configure.help

CONFIG_MAGIC_SYSRQ
If you say Y here, you will have some control over the system even
if the system crashes for example during kernel debugging (e.g., you
will be able to flush the buffer cache to disk, reboot the system
immediately or dump some status information). This is accomplished
by pressing various keys while holding SysRq (Alt+PrintScreen). It
also works on a serial console (on PC hardware at least), if you
send a BREAK and then within 5 seconds a command keypress. The
keys are documented in Documentation/sysrq.txt. Don't say Y unless
you really know what this hack does.