On the left of the source pane are some handy controls to view and control your sources.
Tap the “i” button to toggle the name and source type bar on the bottom of each source.
Tap the square frame button below to enlarge the source pane by hiding the overlay pane.
Tap the two overlapped squares toggles multi-selection.
Multi-selection allows you to select more than one source at a time, which is how you can do picture in picture effects and grouping (refer to tutorial on grouping for more information)
Tap on the two interlocking rings to toggle last layer lock mode.
When enabled, Layer lock mode keeps the last selected source from being deselected. This helps keep you from accidentally cutting in nothing (black video).
Tap the tiny bomb icon to deselect all sources to start building the next shot from a clean slate.
This will remove all sources from the preview pane even with Last Layer Lock enabled.