What exactly is different about this mode?

Your gallery is beautifully trimmed in the Presentation Mode. The range of functions is reduced to the most necessary to avoid distraction from the most important things, your images. What remains is a pure, beautiful gallery view without annoying sidebar, with impressive, large header image, larger preview images, horizontal navigation, large gallery title, individually colored image view and so on. Only the download function is left if you have enabled it in the Gallery Settings.