Hauling is used to increase the line speed, which helps you add distance to your cast.  To accomplish a haul is to simply pull on the fly line during your casting stroke.  A double haul involves hauling on your forward and backward strokes.  This will add some distance to your casts, but it’s not going to make a 50 foot caster into an 80 foot caster.  It can add an extra 5 to 10 feet to your cast depending on your skill level.