Selwyn will show you an example grid with several fields that are held in a container within design mode. For the purpose of the animation, Selwyn has set them up so that they hide unless "Massachusetts" is entered into the field where the state name belongs.
He'll define a button in "field properties" with a show/hide expression, so this container will only be shown if the state entered into the field is Massachusetts. Notice also under "field properties" that in addition to the show/hide expression in the syntax, you'll find the animation command.
In his example, Selwyn has the command set up so that when you show or hide, you'll see a slow-slide animation. But if he were to head into the builder, where the "animation show/hide" should be checked, he can define the animation style and chose how long in milliseconds the animation will run.
Back in working preview, Selwyn will test his expression by typing in something other than Massachusetts into the field. This lets him see that the animation hides. After correcting his entry to be Massachusetts, you'll see the button and container displayed in the slow slide style he chose earlier.