Not sure if you already know this, you can enable it in the keyboard options in the keyboard settings.
System Settings → Keyboard → Key Bindings → Key sequence to kill the X server
Yes it works, if you’ve configured it. If the system is locked up, but the kernel isn’t then it would work. The link below, already posted by @Kobold, tells you how to set it up.
When @Kobold says REISUB, in this case he means the magic sys req key used with f (ie Ctrl + Alt + F), which would call the OOM killer. REISUB would restart your computer.