Auto-Show Apt Diagrams will "zoom into the airport diagram" when you zoom in and out of the map. If the behavior the OP is looking for is, "I want iFly to default to show me the airport when that's where I am and I'm sitting nearly still," then the feature he wants to enable is AutoTaxi, which is on the same settings menu as Auto-Show Apt Diagrams, but two items higher up on the list.