With "Just a Reflektor", you literally have a hand in generating what you're watching; the move of the mouse across the video creates various effects at different points throughout the song using Google Chrome's ability to handle hardware accelerated graphics.
The video is a collaboration between director Vincent Morisset and the Data Arts team at the Google Creative Lab in San Francisco, led by Aaron Koblin. Koblin worked with director Chris Milk for the last Arcade Fire and Google project, a Street View-powered experience for "The Wilderness Downtown", which incorporates the viewer's childhood address to produce satellite imagery from Google's collections into a visual experience.
You can experience "Just a Reflektor" here. (Make sure you are using Chrome as your browser.)
For a more in depth look, here is a screenshot breaking down what your mouse or smartphone can create over the course of the video:
You can explore and experience each effect:
You can also watch this behind-the-scenes look at the creation of the visual music video experience, with commentary from Koblin and Morisset: