Sorry, I forgot to say: the reason this works is your brain has two almost competing halves. The left brain, responsible for analytical thinking, that thinks in lists, and does not want to start anything until it knows everything about how to complete the task - which is a problem when doing creative things, not easily subject to analysis like novels.
The right brain is creative and passionate, and is the side from which writing will flow - writers block is what happens when the analytical side gets in the way and says whoa! You must not do this till you have thought it all through. The write for a minute without stopping and building from that is breaking down the left brain reluctance to commit, to allow the right brain to take over.
As for the 20 - 30 min limit, think that is based in psychology too. Seem to remember it discussed in a book called " brain rules" but I may be wrong on that.
Not many can write for hours without stopping , and almost none can write high quality without editing, although a mystic called Katya (Catalina ) Rivas has certainly done so writing a dozen books without stopping, whilst being critically observed including by scientists, in defiance of the fact she is not an educated woman, but has written in languages other than her own! I suspect the nature of her writings might upset some - but the fact she can is interesting. Perhaps not of this world.