Directing is the process of initiating and controlling movement. This happens mostly subconsciously and automatic, but we can consciously give directions as well. Directions then act as instructions to ourself, and the resulting movement is as efficient as we are capable of in any given moment in time.
Alexander distinguished between general and specific directions, while insisting that the general directions always need to precede specific directions. At the beginning of learning AT, directions are most likely internally verbalised instructions (Let the neck be free, allow the head to move forward and up, let the back lengthen and widen). WIth time, experience and practice the need for explicit verbalised directions diminishes. For the moment, simply assume that a direction is something you tell yourself silently.
The important thing about directing is to refrain from doing anything, yet allow movement to happen. The difference in muscle tension after I directed myself to 'let my neck be free' might not be noticeable - unfortunately, there's no bells and whistles if we do something in an efficient way.
Directions address the quality of movement, and the way parts of our body move in relation to the whole body. Any voluntary movement uses directions, whether we are aware of this fact or not. During Alexander Technique lessons we bring awareness of the directions and the body map used for movement.
Technically speaking, directions are energy impulses traveling along the nervous system. Our intent, manifested in electro-chemical signalling processes, produces the movement of the organism. That sounds probably more complex that necessary, let's take an example to unravel how we use directions to achieve something. If I want to have sip of coffee (intent), I pick up the cup with my hands, lift it to my mouth, carefully tip the cup towards my open mouth to pour a decent amount in, savour the taste and finally swallow the liquid content.
The action of 'picking up a cup' already comprises a highly complex coordination task, which becomes obvious when we observe toddlers learning this useful skill. Our nervous system evaluates the distance between hand and cup, initiates the movement of the arm while adjusting the trajectory to the shift in weight distribution throughout the whole body, adjusts the hands to the shape of the cup, grasping it firm enough to prevent sliding, checking whether the cup isn't too hot to take, adjusts the movement of the cup towards the mouth incorporating the additional weight, moving it smoothly to prevent spilling, tilting the cup so that an adequate amount of liquid enters the mouth.
We don't have to be aware of all of this. Once we acquired and trained a specific motor skill, we can use it intentionally (which can be consciously, subconsciously or a mix of both). Even while applying the Alexander Technique we don't need to bother with the gory details - we direct consciously just the head-neck-back relationship.