A couple weeks ago I predicted that Apple’s virtual Newsstand for iPads and iPhones would provide “a little more convenience for the user, and a little more discoverability for the publisher — but nothing here is a game-changer.”

I stand by the first part of that diagnosis, but it’s now clear there is something game-changing about Newsstand. Since Apple launched it last week in the latest version of its iOS operating system, its impact has been immediate and significant. Many Newsstand apps now rank among the top free apps overall, and magazine and newspaper apps are benefiting from a surge of downloads and subscribers.

Downloads of the NYTimes apps for iPhone and iPad exploded the week that Newsstand was released. (Data courtesy of The New York Times)

The week Newsstand launched, the NYTimes for iPad app saw 189,000 new user downloads, up seven times from only 27,000 the week before, spokeswoman Danielle Rhoades Ha told me.

That’s impressive, but it’s nothing compared to the NYTimes iPhone app, which saw 1.8 million new downloads that week, 85 times more than the 21,000 downloads the week before. Nearly one-fifth of the 9.1 million people who have ever downloaded the NYTimes iPhone app did so last week, with the launch of Newsstand.

National Geographic jumped to the top of the Newsstand iPad app chart, and as of Wednesday is 18th most popular of all free apps. A spike in downloads is great, but for a magazine like National Geographic, the real test is whether those people then purchase a $4.99 issue or a $19.99 annual subscription. They have.

National Geographic’s rate of iPad subscriber growth increased by five times since the launch of Newsstand, President of Publishing Declan Moore told me.

Many other publishers are reporting similar experiences with a Newsstand bump. What’s going on here? In part, discoverability and convenience.

Newsstand has its own section of the iTunes App Store, which makes it easier for niche and obscure publishers to find their audiences. That’s helpful, but I expect the benefit will diminish over time as the 286 current Newsstand apps are joined by hundreds or thousands more.

Newsstand collects all your publications in one place, instead of scattering icons across multiple home screens. It also enables apps to download fresh content “in the background,” so it’s already there before a user opens an app.

But discoverability and convenience are long-term incentives leading to slow, gradual growth. I don’t think they exclusively account for the sudden, exponential explosion of app downloads and subscriptions.

So what is Newsstand’s secret weapon, its viral ingredient? It is, I think, the shelves.

Newsstand's empty shelves.

Empty shelves beg to be filled. Look around your home. Look for all the shelves, in bookcases or perhaps wall-mounted. Are any of them empty? Probably not.

If I went to your house and put up a new shelf today, you would probably find something to put on it by tomorrow. When given a shelf, a human will fill it.

Newsstand exploits this instinct. Its dynamic icon shows what currently rests on your virtual shelves. When you first install iOS 5 or get your new iPhone or iPad, the Newsstand icon’s empty shelves sit there on your home screen, looking lonely. You tap the icon, you see the full-sized empty shelves, and then you see the “Store” button right there to help you fill them. As you fill the first shelf, another empty one appears below it, beckoning you further.

This would account for why millions of people, immediately after getting new devices or upgrading their old ones last week, suddenly downloaded dozens of new magazine or newspaper apps and bought subscriptions they didn’t think they needed a day earlier. They had shelves to fill.

That’s a nice trick by Apple, which understands product design and user experience better than anyone else. I still believe, as I wrote earlier, that news and magazine apps in the long run will sink or swim based upon their individual achievements of “quality writing, crisp design, effortless navigation and a platform-tailored experience.”

But a rising tide lifts all boats, and in this case Apple’s Newsstand has given a high tide to publishers. Now they must take advantage of it.

Correction: The legend in the chart of NYTimes app downloads originally reversed the dates. The green bars are the week ending Oct. 9, the blue bars are the week ending Oct. 16, when Newsstand launched.