It's a plank.
So what's the deal with moving around?
The rotator cuff has two main functions:
1. Don't let the arm get ripped off
2. Don't let the bone fly out the back of the shoulder like a cartoon.
Here, Teresa is getting some core work with the plank and she's making her rotator cuff practice stabilizing against a compressive force while in elevation (feet on box) so that her shoulder blades move and wrap on her rib cage the way she wants in everyday life.