While it is a short movement, it is of high importance that, while doing this exercise, you do the full motion. The motion is basically just in the scapular area, where you srhug while being in a fully extended pushup position.
Upper back
If you work 8 hours in front of a computer like I do, your scapulae are most likely spread apart while you type or even just hold your arms in front of yourself. This is the position that creates a muscle disbalance between your rotator cuff (shoulder) muscles and the muscles between your scapulae and spine are getting streched out with minimal contractions, which created a muscle with a very low tone and not enough strength.
This is why exercises like the scapular pushups (shown on the left) help those trapezius and rhomboid muscles build strength relatively fast and relieve the pain in that upper back area