Berkley has published some very interesting research showing that rhythm throughout the brain may be as important as rhythm within individual areas of the brain. IM trains the brain rhythmically, each task synchronizing different areas of the brain. Here's the article.
One analogy that this paper uses is: "It is like the radio communication between emergency first responders at an earthquake," Canolty said. "You have many people spread out over a large area, and the police need to be able to talk to each other on the radio to coordinate their action without interfering with the firefighters, and the firefighters need to be able to communicate without disrupting the EMTs. So each group tunes into and uses a different radio frequency, providing each group with an independent channel of communication despite the fact that they are spatially spread out and overlapping."
