That's not what the spec says. It's sugested that you should run them above 20% and they're made for high speed, but the what the spec says is that at 20% duty cycle you get 800rpm (+-20%), not that you can't run them bellow that. Also unlike the regular consumer fans they don't advertise fan stop at 0% (though it will likely stop like the majority of fans regardless of what they advertise, mine does well before that). This is pretty standard for any fan, I have the 2000 rpm version and I can start it with no problems at 10% duty cycle (and that's start from 0, not decrease speed), as an example 2 other fans from fractal I have can't go that low without stalling.