Actually, no, that's just the way everyone THINKS they're supposed to do it. I have a Fong Kai FK320 (?) case right now (heavy as a tank!), with a CPU duct system standard issue, and when you receive the case the rear mounted fan that connects to the duct is installed to blow air OUT, the idea is to pull the hot air right off the heat sink & out (it's even recommended to have the cpu fan, if you bother to use one, blow up not down). However, the geek chat boards are filled with messages from a few years ago (when this case was the rage) from people talking about how this fan was installed "wrong" when they got it & thgat they had to reverse it. There was even a published interview with a Fong Kai engineer explaining that the fan was right, but people refused to believe it made any sense.
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OK - here's how I see it.
Put your hand over the top of any of the vast majority of CPU coolers on the market that push air down onto the HS. What do you feel? Air rushing past your hand, on to the HS. Now put a cardboard tube over the top, and put your hand over the other end of the cardboard tube. What do you feel? Air rushing past your hand and down the tube, as before. This is scenario one, with the duct in place an no additional fan. I can't see how it can do anything but draw air in with the majority of coolers out there.
Now take another fan and set it to exhaust air out of the tube i.e. the Fong Kai scenario. What happens? The CPU fan speed increases as air pressure decreases - it's pushing less air, hence less air resistance, hence higher speed. Why? Because it's creating a vacuum effect inside the tube. I can't see how this can make the cooler more efficient, but I'd be happy to be proven wrong - do you have a link to said article with the Fong Kai engineer? I can only imagine he must have been referring to a cooler configuration that draws air away from the HS instead of blowing down onto it, similar to the cases Dell were putting out a couple of years ago. These typically have a shroud that partially covers the base of the HS with a gap at the bottom to allow air to be drawn in. Does the duct in the Fong Kai look like one of those?