The SemanticZoom-control behaves different on Windows Phone 8.1 (compared to Windows 8). Instead of using multi-touch zoom to switch between the two views, you have to tap on the group header to show the ZoomedOutView. From this view, you can tap on an item to switch to its position in the ZoomedInView.
This behaviour is similar to the application list on Windows Phone. The letters a, b, c etc. are the group headers of the ZoomedInView - when you tap on one of them, you will see a list of all letters (ZoomedOutView).
Semantic"Zoom" might be a missleading name on Windows Phone...