Using Presentation Mode
Presentation mode provides a way to gradually reveal your diagram as your present it to other people, so that you don't overwhelm them with too much information at once.
To get started, click the presentation mode icon in the top right of a diagram.
The window will then switch to full screen mode, and presentation mode will start (press escape if you want to return to a normal window, but keep presenting).
Now you can expand and collapse branches by hovering over them, and clicking the (+) or (-) icons. Or, if you're using a phone or tablet, then you can use the icons in the context menu (press and hold on an item) to do the same thing.
You can see where branches are collapsed, and can be expanded, by the dashed lines coming out of an existing item.
If you want to expand a branch to show everything under it (instead of just the next level of items), hold the [shift] key as you click the (+) button!
When you've finished presenting, click the presentation mode icon again to return to editing mode.