Play the animation to see an underline style previewed using live preview and then see a style selected.
Have you ever spent time in the try-undo-try-undo cycle? You select a font, font color, or style, or make changes to a picture. But the option you select turns out not to be what you want, so you undo and try again, and perhaps again, until you finally get what you have in mind.
Now you can see a live preview of your choice before you make a selection. You get better results faster by picking what you want the first time. You don't have to undo and try again.
To use live preview, rest the mouse pointer on an option. Your document changes to show you what that option would look like, before you actually make a selection. After you see the preview of what you want, then you click the option to make your selection.
Click Play to watch the process of using live preview to see how different underline styles will look before selecting a style.