Linux can misbehave, especially on more exotic hardware or with untested software. So, the kernel provides a way to interpret special keys on the keyboard and a more remote-friendly "/proc/sysrq-trigger" interpretation. This program wraps all of that to make it easy to just run a command to get the intended results.
For example, if programs and libc are all breaking but you have a statically
linked sr and can still get a few pages off the disk you may be able to run
sr u; sr s; sr b with some success.
Usage (as root!):
sr
where CODE is:
b immediately reboot without syncing or unmounting
c crash system by NULL pointer deref, leave crashdump if configured
d shows locks that are held
e send SIGTERM to all processes, except for init
f call OOM killer to kill memory hogs; No panic if nothing can be killed
g used by kgdb (kernel debugger)
i send SIGKILL to all processes, except for init
j forcibly "Just thaw it" - filesystems frozen by FIFREEZE ioctl
k secure Access Key (SAK); Kill programs on current virtual console
l shows stack backtrace for active CPUs
m dump current memory info to your console
n used to make RT tasks nice-able
o shut your system off (if configured & supported)
p dump current registers & flags to your console
q dump armed hrtimers (NOT regular timer_list timers) & clockevent dev info
r turns off keyboard raw mode & sets it to XLATE
s attempt to sync mounted filesystems
t dump current tasks & their information to your console
u attempt to remount mounted filesystems read-only
v forcefully restores framebuffer console; causes ETM buffer dump on ARM
w dumps tasks that are in uninterruptable (blocked) state
x used by xmon on PPC; Show global PMU Regs on sparc64; Dump TLBs on MIPS
y show global CPU Registers [SPARC-64 specific]
z dump FTRACE buffer
0-9 set console log level; 0=emergency messages (PANICs|OOPSes) only