It uses a NeuroSky MindWave controller to track Alpha Waves in your brain (roughly corresponding to being in a relaxed/meditative state), and according to these values, it control the next song that plays in Spotify!
If you manage to achieve a highly meditative state, you'll be rewarded with good music: usually a calm, relaxing tune by one of your favorite artists from last.fm. If you lose focus and your meditation level drops, you'll be hit with some of the top-100 worst songs ever recorded!
So keep it chill, dude!
This hack was implemented in 24h. at MusicHackDay, San Francisco, on 2012/02/11-12"