It is undesirable to keep a tight line on a horse. Short pulls and releases will restrain it. A soft nylon or leather halter, compared to a longeing cavesson, may encourage tight line pulling. When the horse is going in a large circle around the trainer with the right tension on the line, the trainer can stand in one position and give the lesson with minimum effort. Changing direction Change direction of the horse and use the same procedure.
Many horses are definitely one-sided in their preference in longeing.
Work the weaker side more than the stronger side until the horse will longe in both directions in good form. Horses are often frustrated when direction is changed. Sometimes they try to escape by running backwards. See that they do it right in the beginning. A horse may want to run while being longed.