To demonstate this feature change the width of the browser window. As the width becomes too small to fit all controls, the groups will collapse to save space. Click on the collapsed group to show the group's controls as a pop-up. Click away to dismiss the pop-up. As the browser width increases the groups are expanded again.

When the width of the browser becomes too small to fit the collapsed groups the whole Ribbon is hidden. The Ribbon will be shown again as the browser width becomes wide enough.

By performing a post back you can see how changes made to the Ribbon on the client are reflected back on the server.