SCNView provides substantial default mouse/gesture handling, but I can't find a description of what various things do. For example, holding the Shift or Option keys while moving the scroll ball (on a Mighty Mouse) vertically zooms in and out, but in different ways. One seems constrained, the other does not. Is one moving the camera in and out, the other changing the FoV?
Similarly, in Xcode, the SCN viewer has several modes (e.g. Fly, Turntable, Arcball), but it's not clear to me how those differ, nor if it's something provided by SCNView.
