Rolling a door is a aligning the door with the fender without going straight up or down. Since the fender and the door are not flat, they are rounded, you have to roll it. Like if you were trying to line up a line on two halfs of a ball. If you roll it you can line up the body line without dropping the door. You have to use all your hinge adjustments. I like to hook a strap to my overhead trolly so that the door doesn't 'fall' when I'm adjusting. If you get the fender/door seam perfect and the door/quarter seam is a little off you can actually twist the door by hand to get the back contour to fit the quarter. This is a last step/resort to get line up. This is tough to describe with actually having a car to show you on.