Here’s a mental picture: It’s raining, and you’ve decided to lift outside (not a good decision, mind you, but it’s too late now). Staying over the bar means that, from the start of the pull (i.e., your setup position) until just after it passes your knees, you want to stay leaned over so that your shoulders and upper back are keeping the middle of the bar from getting rained on (your shoulders will actually be slightly in front of the bar). If you raise your torso too soon, the bar gets wet, and that’s no good.