I was watching this video '300 actions a minute? Truly mastering StarCraft'
And I'm bewildered at how fast they are playing. How is this possible? I can't even think about my next move so fast, let alone be typing shortcut keys that fast.
It's a combination of the keyboard and mouse. I'm sure we've all used shortcuts on our keyboards and that's most likely what the gamers do - use shortcuts. If you think about the beginning stages of the game: you click on your hatchery or whatever, build a drone, you can use a keyboard shortcut to build the drone for you. Then you click on a drone, tell them all to go collect minerals and then you click on one drone and tell it to build a spawning pool (also a shortcut).
We probably all use ctrl+c , ctrl+v pretty quickly - the gamers have just, like McKay said, tuned their brains to all the shortcuts for Starcraft.
You know how some people can type 100 words per minute? That's more than 300 actions per minute. The pro SC players have just wired their brains differently than your traditional secretary.
Practice. Lots and lots of practice.
And the implant of course. :-)
APM is primarily limited by what you can remember to do, not what you are physically able to do.
McKay's secretary example is also correct, when typing you generally know which key you are going to press next. If you always know what you are going to do next in Starcraft 2 your APM will skyrocket.
If you look at the rest of the documentary (http://vimeo.com/12730027) you will notice how much of their life they pour into this game. It's not only that they spend their entire life doing it, but only select few get that high that it appears to be a combination of practice and natural skill.
It is not much unlike other sports where people spend a long time training and still only some excel.