Prone Y Raise

You go too heavy, so the tops of the shoulders take over and the arms never form a clean Y.

You shrug toward the ears on the way up, which switches the load from the lower traps to the upper traps.

You swing the arms up with momentum and let them drop straight back down.

You lift the chest off the bench and arch the lower back to gain height.

Draw the shoulder blades down and back first, and let the arms rise as a result of that squeeze.

Keep the thumbs pointed up and the arms in a wide Y, not straight overhead, so the lower traps do the lifting.

Keep the neck long and the chest on the pad; a light dumbbell with a slow lower is the whole point.