Ushiro shihonage vectors

Tonight in class Ben Sensei helped me with this. He said that Uke needs to be extended, and we tried various ways to do this with a non-compliant Uke (Andy and Sensei). First, from looking at this still I can see what I typically do with my feet. I tend to move my foot along the vector with the big red X on it instead of the other one. In other words, I try to pull Uke‘s arm. I know I want to avoid pulling. However, we tried a few atemi tricks to deal with stiff Uke‘s, but I still want to do this beautifully. I feel I need to get Uke off balance more by pressing on his forearm to lower him, then move his arm along with my core. I can finish with my inner-elbow pressing trick to complete a safe shiho-nage.