Amazon Underground takes paid games and gives them away for free, with all the extra in-app purchases included. For users, it's a tempting way to get around handing over money for games you really want to play. The trade off comes in the form of ads and privacy. Amazon keeps up with every moment you spend gaming, because this is how it determines the amount of money to pay developers.

Rovio, the maker of Angry Birds, says it has tripled revenue for four of the games it has published on Amazon's app store by joining Underground. These four games—Angry Birds Rio, Angry Birds Slingshot Stella, Angry Birds Space, and Bad Piggies—have been available for free since Amazon first launched its new program.

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That said, Rovio hasn't disclosed just how much (or little) money it was making before the change. Still, it makes sense that the game developer would see a spike from Underground. Angry Birds is a time suck, and a few players can rack up long play times pretty quickly. Amazon's formula rewards developers who get users hooked on a game, regardless of the quality or depth of the experience.

But time may yet show Amazon to have a winning formula for combining casual users with casual games in a way where both sides get what they want.