I think it will trigger a multi-touch gesture when held on the phone with the app open.
(Cut & paste from my post on G+ https://plus.google.com/103850062796903816685/posts/WNaytFhE...
It includes a card which it instructs you to "hold here" and hold up to the screen of your phone. At first, I thought this couldn't possibly make any sense: NFC isn't super ubiquitous and works on the back of phones. It wasn't telling you to hold it up to the camera, and besides, there's nothing obviously machine-readable on the card. The card isn't transparent and didn't look like it would "line up" with anything on screen in any meaningful way. But, I figured they must have something up their sleeves.
Opening up +Markers, and holding the card up against the screen as directed yields 5 spots. They have some sort of conductive element in the card creating a multi-touch interaction. I'm guessing once the real app comes out, if you do this, it will recognize the particular spacing/configuration of this 5-touch gesture and do something interesting.
It's clever in that it should work well across a broad variety of devices, but is hard or impossible to accidentally trigger by hand, depending how precise the tolerances are on the spacing of the 5 points. I look forward to seeing what the app actually does with it.