The pineal gland is a pea sized, pine cone shaped organ situated at the center of the brain, directly behind the middle of two eyebrows. Physiologically it produces neurotransmitters melatonin and serotonin that are responsible for the sleep cycles as well as moods, respectively.
It is also known as the third eye as it contains photoreceptors like a normal eye.
All cultures have described it in various ways as the seat of the soul; responsible for spiritual enlightenment; a sixth sense which sees beyond what the five senses can see; a symbol of spiritual awakening; a connection between the physical and spiritual world. In the Indian context, the space between the eyebrows is called Agya Chakra and represents the pineal gland.
This background will help us understand Krishna's method to control senses and mind when he says, "Shutting out all thoughts of external enjoyment, with the gaze fixed on the space between the eye-brows, equalizing the flow of the incoming and outgoing breath in the nostrils, and thus controlling the senses, mind, and intellect, the sage who becomes free from desire and fear, always lives in freedom" (5.27-28). This is a method or technique given by the Lord to Arjun to help him control his senses, mind and intellect.
Vigyaan Bhairav Tantra contains 112 such methods given by Lord Shiva and one such technique says, "Concentrate without thoughts on a point between the eyebrows. The Divine Energy breaks out and rises above to the crown of the head, immediately filling one completely with her ecstasy."
Pain is an automatic tool to bring our attention to the injured areas and this helps us in survival. Similarly, it is bringing conscious attention to the area between the eyebrows to activate the pineal gland and this activation will fill us with internal ecstasy without the help of any senses.
Source - Daily World