About a month ago I bought a game called Whale Trail during a sale on Google Play. Today, when I tried to play the game on the subway, it asked to verify the game's license, and failing to do so (since there was no internet connection on the subway), shut the game down. When I got home, I took a look at the game's permissions tab and saw this:

Market license check Can check if you have a license for this application from Market

It seems like this is a feature built into Google Play. So my question is, how does this "license check" work? How often does apps using this "feature" phone home to verify that indeed I'm not a dirty pirate stealing these developer's apps?