Based upon that video.... although it's difficult to see....
He's got two layers for each side of the desk. These two layers are linked.
So although he clicks one layer and drags, both layers move.
The layer controlling the tabletop, has a layer mask on it. And that layer mask hides or shows content when he drags.
It's nothing overly special or complex. You just have to pay attention to the little link icon that appears when he highlights a layer, then watch the mask on the lower layer move when he moves the mouse.