The setup is simple, really—click on a game and music begins to play. In what's actually a nice touch, you don't choose from music from the game; each game is more akin to an internet radio channel than a YouTube playlist. But that's nice! I like not having to worry about which track I want to listen to next sometimes and just let the tunes flow.
Awesome, there is also a knob for "emotion." Here's hoping they add more games in the future.
(Note: I got this site working fine on my PC but had some issues with it on Mac. It's worth giving it a shot in a few different browsers.)