Begin straddling a line with your feet shoulder width apart.  Maintain a slight bend in the knees and keep your weight on the balls of your feet.  Move your feet in and cross them over so that one foot ends up in front of the other.  Try to bring your feet in far enough so that you are completely over the line.  Move your feet back out to shoulder width and bring them back in, this time with the opposite foot in front of the other.  Continue this movement pattern for the designated time, moving as quickly as possible while maintaining control.