How is Workout app UI possible?

I am analyzing how Apple's own Workout app is working and I can't figure out how they do it.


1. Base UI is table, where you can tap a row

2. This presents a countdown timer

3. Timer then animates down to reveal a 2-page UI


What I don't get is how it's possible to load 2 pages without using reloadControllersWithNames:contects: call? That call is pretty jarring visually when I try it so either there something I'm missing here or is Apple using a private API to do this.


I don't see how it's possible to move from one controller to 2 side-by-side controllers in the way Workout app does it.