Brahmarandhra is a technical term in Yoga, indicating an aperture in the topmost portion in the skull, which is said to lead to Brahman, i.e. the Absolute, if the vital air (prana) passes through it at the time of death.
This fact is brought out in Katha Upanishad (II.3.16) where it is said that, “from the heart one hundred and one channels (nadis) emerge. One of them goes to the top of the skull (murdha). If the vital energy goes out through it, leaving the body, then immortality is achieved.
In Taittiriya Upanishad (1.6.1), two names are used for that aperture, namely indrayonih, and kesanta. It is described as the aperture at the topmost portion of the skull. It is said to be the door leading to immortality.
In Darshana Upanishad (VI 32-42), the technique of Sankuhi Mudra is described. An adept can achieve immortality through it by conquering prana in the Brahmarandhra.
In Hatha Yoga Pradipika, many words synonymous to brahma randhra are mentioned. They are sushumna, sunya padavi, muhapatha, smasana, sambhavi and Madhya marga. Practice of mudras of Yoga is recommended here for arousing the dormant kundalini, which remains in a sleeping state at the base of the sushumna nadi. (Brahmadvaramukha). The technique of ten mudras of hatha yoga is described after mentioning the above fact (111.4-5). The arousal of the kundalini, making it rise through the sushumna, and merging it with the Lord in the lotus of thousand petals (the highest chakra), is the procedure for attainment of immortality through Brahma randhra.