At the bottom of the Command Bar you'll find a new interactive section we've internally been calling the "chin". Because the Command Bar is designed as primarily a keyboard-driven interface, a lot of the available functionality is a bit hidden. For example, how does someone learn the shortcut for going back in the Command Bar?
The new UI adds additional structure to the Command Bar that makes functionality easier to learn and discover. You'll find buttons for navigating history, an area for extra keyboard hints to show up and a "settings" button to discover extra operations for the view you're on.