ok, I must admit it took me quite a while to get it. After reading the documentation I found THE sentences to define it in a few words right on the landing page:
You will find it on http://labs.live.com/volta ...
This technique sounds more than interesting. It gives you the opportunity to delay several ground shifting decisions within your development process to a later stage. Especially in a phase when you are not sure on which directions things will take in the end this is cool. Another thing is that cross development involving servers and clients (and sorry todays webpages are no longer thin, dumb HTML) has its burdens. The ability to develop first, test and show around then break apart...sounds smart to me.
This delay of decisions is one thing I read quite frequently about what makes Ruby so smart...
What I also like about this approach is that it does not neglect the fact that there are differences between the two models (one tier vs. multi-tier). But it lowers the step significantly.
Have a look at it.