The number of resistance cords you use is determined by the exercise and your workout level; beginner, intermediate, or advanced. The workout DVDs and workout chart that come with each reformer specify the number of resistance cords to use with each exercise. For some exercises less is more challenging. In general more cords engage your mobilizing muscles while fewer cords work on your core. Use the number or combination of resistance cords that challenge you without compromising your form. If the exercise is too easy, add a cord, if it is too hard, take a cord off and see / feel what the difference is. The same applies when using the cardio rebounder.