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. Software that has the potential to completely lock up the system, so that a BIOS interrupt is the only input that could be generated, use SysRq as a form of "panic button." e.g., the linux kernel can be compiled with "Magic System Request Key support" (without using the BIOS, most likely).
See also scroll lock, break key.