- Alt-SysRq-S asks the kernel to sync all filesystems.
- Alt-SysRq-U unmounts all filesystems (actually, remounts them all readonly)
- Alt-SysRq-B reboots the system immediately
Many vendor-compiled kernels have the compile-time flag on, but disabled at runtime via /proc/sys. Re-enable it with "echo 1 > /proc/sys/kernel/sysrq", or put "kernel.sysrq = 1" in /etc/sysctl.conf.
Alt-SysRq-S,U,B (hold down the Alt-SysRq while you sequence through S, U, B) is a safe reboot with no fileystem damage, in cases where the system appears to be locked up, but the kernel is in fact still alive enough to see your SysRq requests.
See here: Linux Sysrq docs for more details and all of the other sysrq functions available.