When you open an “Active Card” by clicking “Read More” button and you scroll the Content Timeline to the left or right, next item that comes to the center of the timeline will also have its “Active Card” opened.
The default state of Content Timeline for this option is “enabled”. This means that by default the next scrolled card will remain opened thus creating a nice looking transition between the items.
By ticking the “Close item on transition” checkbox the “Active Card” will be automatically closed once you decide to scroll the timeline left or right.