Single Step is a display mode design for animations. Turning it on only the active layer is visible. Activating another layer automatically hides the last one. This way - especially in combination with Activate Previous Layer (Ctrl+Shift+Tab on Windows and Command+Shift+Tab on Mac OS) and Activate Next Layer (Ctrl+Tab on Windows and Command+Tab on Mac OS) - it is possible to switch very fast through all steps of an animation and view them.