At 6:10 Harini is telling us how to handle a specific "is this phone" UI. These are referred to as tailored experiences. The basic idea is that you don't; instead, you determine when you want to have a one-handed experience and to query the diagonal size of the physical device. This is not a visual state solution. This is a query on load and a separate XAML view.
Where do we find more information about the XAML binding engine? I work with WPF and would love to see if it could benefit us. Currently we do our own custom way of data binding and relay commands. Thank you so much for the new development of such a flexible technology!