Open-chain motions always look different from closed-chain motions. But in the case of scapular depression, the two differ greatly! As always, open-chain movement involves the distal end of the chain moving in relation to the proximal end. The scapula, taking with it the rest of the upper extremity, moves downward - as in reaching downward toward the floor in an upright sitting posture. Notice in the animation the lack of a muscle contraction since in the upright position, gravity pulls the scapula downward through depression.
The closed-chain version involves movement of the proximal end of the chain in relation to the distal. Since scapular depressors (latissimus dorsi, lower trapezius) attach proximally to the trunk, the trunk serves as the proximal end of the chain. In closed-chain scapular depression, we stabilize our hands on the arm of a chair or the surface of a mat. As we push downward, the scapulae, upper extremities and hands cannot move downward, so the depressors' shortening contraction pulls the trunk upward instead.