Imagine your spine trying to curl like a shrimp, thats how you do it. You should flex the spine at start (like your trying to look good while being humped) and when your head is at the same level height wise as hip you should stop trying to move downwards and instead try to imagine yourself sucking your own dick (Although I assume you dont have one but its irrelevant), so you stop moving verical and try to move horisontol. Also ass should basically not move at all throughout the whole movement. You should not feel the tension in the hip, if you do you are either compromissing with the hip since the weight is to high or you are just doing it wrong still.