Shoulder blades are not the same as shoulders
Two things can happen at once: an arm moves, and a shoulder blade moves with it.
Observe arm-to-trunk motion separately from shoulder-blade motion on the rib cage.
A small tabletop exploration separates protraction from retraction without turning it into a full Cat-Cow movement, then shows elevation, depression and rotation of the shoulder blades. In the Down Dog example, label the arm relationship and the scapular action separately.
Shoulder flexion and scapular protraction are not competing answers to the same question.
Arm movement and blade movement
The arm changes relative to the trunk while the shoulder blade moves on the ribs.
This is a simplified learning diagram. It is not a reviewed anatomical illustration.
Downward-Facing Dog
Select "Arm relative to trunk" or "Shoulder blade on ribs." Play one movement pair at a time. Then return to the shoulder close-up in Down Dog.
Let your arms hang, then draw your shoulder blades toward each other and let them go.
Notice: The arms went along for the ride. The movement belonged to the shoulder blades.
This is noticing, not instruction. Move as little as you like, stop at any point, and watching instead of moving is a complete way to take part.
Which label describes the shoulder blade, rather than the arm?
Pause before opening Practice. Say the distinction in your own words.
What changes when the blade wraps around the ribs but the elbow stays straight?
Do not infer scapular rotation from a poor side crop. Supply a clearly identified posterior-oblique detail view; no fixed universal 170-180 degree requirement.