The key is not totally inactive, however. When it is pressed, nothing is stored in the keyboard buffer, but a BIOS function is called. The default handler of that function does nothing and simply returns. Programs can use SysRq simply by creating an interrupt handler to replace the default stub, but most programs have no need for that functionality. When software that has the potential to completely lock up the system is run, a BIOS interrupt is the only input that could be generated; the SysRq BIOS interrupt handler can then be written as a form of "panic button" to gracefully terminate the program or reboot the system.
In Linux systems, provided the kernel has been compiled with the correct option, the key can be used to perform a variety of functions in an emergency, such as syncing disks, killing processes and powering off the computer.
See also scroll lock, break key.