When you do first pull, that is to drive the bar up and get it moving, pull your shulders back at the start. As you reach the top make sure you hit lockout. Pull your shpulders back and a slight hip thrust forward as you squeeze the glute hard. Keep the bar dragging against the legs the whole time. If done correctly you will have racing stripes all the way from your shins to the top of your thighs. Keep pulling with your shoulders. The sequence or as I like to call it is Bar before butt. This means the bar should leave the floor before you start your butt in motion.