(This is for my first post wrote in English, I'm already sorry for all the mistakes you would find in it)
I'm currently developing a new app, HFR RT, in which I wanted to display a Minimized state of my AppBar, à la Windows Phone.
Note that the new Mail and Calendar apps included in Windows 8.1 have the same behavior. In my opinion, we should find this behavior in every built-in app, and Microsoft should provide an API to do that.
Anyway, here's this little piece of XAML that you can put into a UserControl or directly in your Page.
Don't forget to name your page; if not, the Binding will not work (here the page is named MyPage).
{% highlight xml %}
Then, you just have to add this line in the Tapped event:
{% highlight csharp %} BottomAppBar.IsOpen = true;
That's it!