It's not the transition from one layout to another that is graceful. Under normal circumstances, you're not resizing your browser to use a site. What's graceful is the fact that the layout is well suited for each and every resolution you throw at it, whether you're on a full-screen desktop or a mobile phone.
They should release a video of shim in action. A video explains it much better than words can. (I was lucky enough to get a sneak peak of all their new stuff a little while ago.)
