Via linuxtoday.com, I found this article.
But it can be hard to memorize.
what’s the magic? Well if you are ever stuck with an unresponsive system try
Pheew! So how many hands do you need to do this? Or should you use your toes? Well, actually you only press and hold Alt and PrintScreen together, the other keys are pressed one at a time and then released. There is quite a lot happening when you do this.