(If the focus on how many people watched the commercials sounds a little peculiar, think of it this way: if you were spending tens or hundreds of millions of dollars on TV, wouldn't you want to know if people were actually watching your ads?)
As National Geographic, and other advertisers have found, there are myriad factors -- other than the content of the ads themselves -- that affect viewership of commercials, ranging from what type of program it is to what time of day it is. The trick is to expand upon those variations with techniques that keep people from running to the bathroom once the commercials come on. In National Geographic's case, the channel has run commercial pods which incorporate talent from the show into the commercial break but not necessarily in the standard product endorsement fashion. The story explains that the star of its "Dangerous Encounters" series explained during a commercial break what the behavior of geckos is liked, sponsored by ... well, if you can't guess, you really haven't been watching enough TV.
At a certain point, all of this audience manipulation can get profoundly creepy. National Geographic cops to hiring a company "which specializes in looking at the physical reaction viewers have to various elements displayed on screen" to help its C3 cause. Still, this piece is a helpful guide to how the TV industry can increase its C3 ratings, and since it has to make money, manipulate away.