IWhen my taiji partner was teaching me about peng, he kept telling me to pretend I was crushing an egg between my shoulder blades and pushing the resulting mess down into the ground, again with my shoulder blades ("scoop back and down"). My inner PT translated that as "retract and upwardly - yes, upwardly - rotate the scapula" and low and behold my arms went up, elbows first. My deltoids of course were working - but they weren't operating in isolation.