Not sure what you mean by this. Mobile first just means everything is designed starting from a mobile view. It's not a "feature" like some sort UI item, it's a style of design thinking with the goal of ensuring the mobile context is absolutely as simple as it can be. It's not a buzzword since this is how they approached this version.
Just check out the css, everything starts with the 480 media query and builds on top of that. CSS is added to the base mobile CSS (which is incredibly simple) to work as the width goes up. Check v2, it's (almost) completely the other way around. Again, it's not a specific thing, it's everything.
