Ideally, you should not move your body – this way you will isolate the muscle more, improving the effectiveness of the exercise. Also, it is said so in order to prevent injuries – those unexperienced may get injuries by moving too much. Also, the fact that you need to help yourself with the body means that the weight is excessive for you. Don't look all that much at pro-bodybuilders, as they push themselves to extreme limits, and they try to get the benefits of the negative reps, rather than positive.