I have on my desk a rare and valuable document. It's an early user manual (PDF) for the Kobo Aura HD, and while it doesn't offer a complete explanation for the Aura HD's features it does drop a hint about the unusual design for the rear of the shell. According to the diagram on the 8th page of the manual, the original design of the Aura HD may have included some type of page turn buttons on the back:
Apparently the double crease in the backside was intended to create a protected spot where Kobo planned to put page turn buttons. It's not clear what tech was supposed to go there, but the manual does reference swiping and tapping as ways to turn the page. That is probably a sign that some kind of capacitive sensor was supposed to be mounted there.
I'm just speculating on how it would work, but one thing I can tell you is that the production model doesn't have the buttons/sensors. I think it would have been prominently mentioned in the reviews, the official user manual, and in the launch press release. I also can't find the sensors on mine, nor have they been mentioned on MobileRead.
So how do I know that the page turn buttons were supposed to be there? It's because Kobo submitted a copy of the user manual when they sent in the necessary paperwork for FCC certification.
As you probably know, Kobo sent the Aura HD through the FCC in March. They took the usual step of placing everything interesting under an embargo (pretty much everyone does this). That embargo was lifted a few days ago, and we now know lots more about the Aura HD than we did before, including what it looks like inside (there's an internal microSD card slot). And thanks to Kobo's oversight we also know what was supposed to be on the backside.
It's a pity that Kobo left this part of the design unimplemented; it's a novel concept that is worth exploring. The overall trend in ereader design has been to drop page turn buttons from the design, and the Aura bucks this trend by moving the buttons to a new location - right underneath where your fingers were going to be anyway.
I think the concept is a great idea, and I wish we knew how it would work out in practice. Unfortunately we are probably going to have to wait until the Fall before we can find out. That's when I am expecting Kobo to launch a new and improved Aura HD, along with their other new ereaders.
P.S. Speculation as to why the page turn buttons are missing will be left to the comment section. Would anyone care to take a guess?