I have a Koker but it's direct vent so I've never adjusted the combustion intake. I use a rheostat to adjust the speed of the draft motor.
It seems to me what Keystoker is saying is when your draft gets down to .02 the combustion motor is putting more air into the furnace than the draft can pull out. The difference between the two ends up leaking out of the furnace as CO. So closing down the combustion motor opening will reduce the air being pushed in to something the draft can handle. So by doing this you're not increasing your draft, you're reducing your combustion, which will result in more unburned coal or maybe even having the fire go out. But it will also reduce the chance of putting CO into your house.
That's what I think their instructions are based on.