The First chapter of the secret of the golden flower explained in detail the practice of opening the third eye. The third eye is located around the position between the the eye-brows. It is however, not a physical concern, not on the surface nor located physically inside. The third eye is an aperture (竅) opens only when a particular meditative state of mind is active. Psychologically, according to the text, requires, as a special technique, faith (誠).
If it sounds mythical, it is actually not. For the text spelled out another special technique, 3-points at the tip of a triangle. The 2 points at the base represent the eyes whereas the third point represents the third eye.
How to establish the third eye by faith? First is a mental state between sleep (when the unconscious rules) and wake-up (when consciousness rules). And send it is established through the balancing subtle energy of the two physical eyes). The third eye, being non-physical seems to have no use, but its useful is established through subtle energy establishing its existence. And after its existence is being established, in subsequent practice sequence, it (representing "the essence of human nature - 元神)acts as the control centre of the whole meditative practice.
In Chapter 11 of Dao De Jing, there is an example of powerful use of "nothingness" that is established by physical elements: 三十輻，共一轂，當其無，有車之用, thirty spokes establish the centre hub, it does not exist (as a physical element), but make in a wagon.