Leg Aids

Just as with the reins, the legs can give sending or drawing messages. A touch close to the horse's body gives a 'send' message whereas a loosening and slight removal of the leg to the side can lead(or draw) the barrel and hindquarters into that direction.
A sending aid is pretty straightforward while a drawing aid is more complex and very often will need to be taught to the horse or sensitized in him as he will need to detect whether your intent is that he move into this open leg or if the open leg is just trying not to incite him in any way.
Young horses are well aware of how to come into your space. They usually need to be shown how to move away from you. Many trainers only want the horse to understand this 'moving away'. I much prefer that the horse be well versed in the messaging of how to move away AND move toward the leg upon my request. This makes training considerably much easier later on. When, for example, you want the horse to step to the right, you might ask with the left leg but you will need to check some of the forward motion with the reins and seat. Add the concept of a drawing leg on the opposite side and you clarify what it is you want--step over--step toward my open leg.