The exact meaning of Sanskrit word Antarmukham is inward journey. Our mind and Consciousness withdrawing towards its origin is inward journey. Outward journey is, these two joining with nature going outward into the world.
Every day, we experience both of these states. We become internal during deep sleep and external in the waking state. However, we need to observe the changes happening in our breath and intellect during both these states.
In reality, does breath depend solely on external air? We can understand everything; if we think about any physical, chemical, and intellectual changes happening. Normally when we breathe, oxygen from the external world enters the lungs and reaches tiny air sacs, surrounded by capillaries, which are small blood vessels that take the air and expel Carbon Dioxide thereby purifying blood. The world is aware of this fact.
There is another unknown truth concealed in this process, and that is the consciousness in us. It travels from our brain through our spinal cord to the other nerves and moves outward in exhalation. I.e. it is trying to reach its origin in the brain going outward in exhalation and inward in inhalation. This consciousness is the life force (prana shakti) in us. Our life force or vital energy has five divine qualities, they are divine energy, divine wisdom, divine bliss, divine light, and divine heat. The quantity of life force is directly proportional to the level of consciousness.
Though; humanity is well aware that exhalation is more than inhalation in every living being, no one is particularly observing that. We are living using our consciousness for other activities thinking it is natural. Here, we need to observe that, the life force in us is different from the oxygen in the external air. Our life force is associated with consciousness. We are living with it only. It is moving up and down, coming down from soul in us and going back upwards. If this life force leaves we die. Though there is Oxygen always available outside, why do we die? That means we are not living only with this chemical inorganic gas. We can use Oxygen by filling it in a cylinder and transporting it to wherever needed, it only acts as a catalyst aiding blood circulation. Therefore, it only helps the movement of life force in us. If we have at least a minimum amount of life force, we can survive with external oxygen. However, if our life force falls below a minimum, no amount of external oxygen, blood transfusions, or artificial respiration through a ventilator will be of use.
Therefore, our primary objective is to increase the amount of life force within us. This process of increasing is known as Pranayama. Prana + aayaamam = Pranayaamam. Aayaamam means extending or elongating. That means elongating the life force that is going upwards a little more. There is no need for external air for this. However, there is a connection with external air till our life force is elongated to the seat of the soul in the center of our eyebrows ie; as our life force ascends upwards the connection with external air diminishes. Once the life force merges with the soul even breathing is not involved. This is called samadhi. Life force withdraws from ida pingala nerves and unites with the soul passing through the central canal of the spinal cord (sushumna). This is the sacred union of three nerves (triveni sangamam).