People tend to use their upper traps too much when focused on rotator cuff work as the arm moves overhead. The middle and lower trap are important during overhead exercises to promote optimal mechanics. More importantly, we have to learn how to activate the trapezius as a stabilizer muscle during strict glenohumeral joint motion.
Here is a great exercise increasing mid and lower trap muscle activation. According to Maenhout et al 2016, the highest middle trapezius and lower trapezius muscle activation was found in prone exercises with greater than 80% maximal voluntary contraction. Why is training the scapular stabilizers at a 90/90 degree shoulder abduction and external rotation so important?
Well look how familiar this position is, think of a pitcher throwing or a CrossFit athlete performing a snatch. The shoulder is VULNERABLE in this position, due to risk of instability of the anterior/inferior glenohumeral joint capsule. Also the labrum is speculated to be weak in this position. SO we have to train our scapulae stabilizers and rotator cuffs to be strong in this position!
Be sure to avoid shoulders to ears and keeping the elbow relatively stable with little movement during this exercise. I tried to exaggerate too much upper trap in the first rep!