Imagine there is a thin, straight line extending in front of you and down the path you walk. (If you go to a track, use one of the lane lines.) When walking at a pedestrian pace, without using any race walking techniques, each foot will land on a different side of the line.
Then, as you increase your pace a little, your feet land just on the edge of the line.
In contrast, when you race walk, your feet land in an almost exact straight line. After you learn to use your hips efficiently, your foot placement changes slightly to imitate this near straight-line placement. But please beware: when you try to mimic this action without using your hips, you place an unneeded stress across your knee.
Never allow your feet to cross over the line as depicted below.